ill... 


Edmund  O'Neill 


QUALITATIVE    ANALYSIS 


A  MANUAL 

FOR   THE   USE  OF  STUDENTS  OF  CHEMISTRY 
IN  SCHOOLS  AND  COLLEGES 


BY 

L.  M.  DENNIS 

PROFESSOR  OF  ANALYTICAL  AND  INORGANIC  CHEMISTRY 
AND 

THEODOEE  WHITTEI^EY 

INSTRUCTOR  IN  ANALYTICAL  CHEMISTRY 

CORNELL  UNIVERSITY 


BOSTON,  U.S.A. 

GINN  &  COMPANY,  PUBLISHERS 
&t&en*tim  JJreaa 
1902 


Pf 


COPYRIGHT,  1902,  BY 
L.  M.  DENNIS  AND  THEODORE  WHITTELSEY 


ALL  RIGHTS  RESERVED 

IN  MEMORIAii 

' 


PREFACE 


THE  purpose  of  the  authors  has  been  to  prepare  a  work 
on  qualitative  analysis  that  shall  be  both  exact  and  compen- 
dious, avoiding  on  the  one  hand  the  diffuseness  of  the  larger 
treatises  and  on  the  other  the  incompleteness  of  the  smaller 
manuals.  Full  statement  has  been  made  of  the  behavior  of 
the  bases  toward  certain  reagents,  but  extraneous  details  have 
been  omitted.  Under  the  headings  Methods  of  Analysis  explicit 
directions  have  been  given  for  the  separation  and  detection  of 
the  elements,  and  these  directions  are  followed  in  every  case  by 
a  full  discussion  of  the  reasons  for  the  different  steps  and  of  the 
difficulties  that  may  arise  in  the  course  of  analysis.  It  is  hoped 
that  this  method  of  presentation,  together  with  the  occasional 
references  to  such  articles  in  chemical  journals  as  deal  with 
debated  points  or  new  methods,  may  serve  to  arouse  the  inter- 
est of  the  student  and  to  discourage  on  his  part  a  blind  and 
mechanical  style  of  work  that  is  so  common  in  courses  in 
qualitative  analysis  and  is  so  greatly  to  be  deplored. 

The  Introduction  discusses  in  considerable  detail  the  prin- 
ciples and  operations  involved  in  qualitative  analysis,  but  it 
does  not  include  the  consideration  of  the  dissociation  theory. 
However  valuable  this  concept  may  have  proven  to  chem- 
istry, the  authors  feel  convinced  that  it  should  not  be  made 
the  basis  of  instruction  in  qualitative  analysis,  and  that  its 

iii 


iv  PREFACE 

consideration  may  profitably  be  deferred  to  some  later  time  in 
the  education  of  the  student. 

The  method  of  balancing  equations  described  on  pages  8-11 
of  the  Introduction  was  suggested  to  one  of  us  (L.  M.  D.)  in  the 
year  1890  by  Professor  Gerhard  Kriiss  of  the  University  of 
Munich,  and  it  has  been  used  in  the  laboratory  of  Cornell 
University  since  that  time. 

The  method  of  instruction  used  by  the  authors  is  given  in 
page  3  of  the  Introduction,  the  preliminary  study  of  the  ele- 
ments and  groups  as  there  outlined  being  then  followed  by  the 
application  by  the  student  of  the  knowledge  thus  acquired  to 
the  analysis  of  mixtures  whose  composition  is  unknown  to  him. 
Most  of  these  mixtures  are  given  to  the  student  in  the  solid 
form.  If  limited  time  or  other  reason  makes  it  desirable  to 
shorten  the  course  as  above  outlined,  time  may  be  gained  by 
omitting  the  first  part  of  the  course  and  proceeding  directly 
to  the  study  of  such  preliminary  experiments  as  are  printed  in 
heavy  type,  following  these  with  the  study  of  the  analysis  of 
the  different  groups  and  of  the  detection  of  the  more  common 
acids,  and  concluding  the  course  with  the  analysis  of  some 
mixtures  of  unknown  composition. 

In  many  operations  it  is  desirable  to  use  acids  of  six  times 
normal  strength.  The  formulas  of  acids  of  that  strength  are 
printed  in  heavy  type;  thus:  H^SO^.  Wherever  the  strength 
of  the  acid  is  not  designated  either  in  this  manner  or  otherwise, 
dilute  acid  (twice  normal)  should  be  used. 

L.  M.  DENNIS, 
THEODORE   WHITTELSEY. 
ITHACA,  NEW  YORK, 
September,  1902. 


CONTENTS 

PART   I 

INTRODUCTION 

PAGE 

QUALITATIVE  CHEMICAL  ANALYSIS         ......  1 

QUANTITATIVE  CHEMICAL  ANALYSIS           .....  1 

REACTIONS .         .         .  .         3 

EQUATIONS    .                  .'        ...         .         .         .         .  7 

PRECIPITATION          .                 x»"      •         •         •         •         •         •  •       11 

FILTRATION.         .         .         .         .         .         .         .         ...  14 

SOLUTION  OF  THE  PRECIPITATE      .       •.         .         .         ...  .       16 

EVAPORATION       ...        .        .        ...        .        .        .  17 

PART   II 

THE  BASES 

HYDROCHLORIC  ACID  GROUP  .  ....  .  .  .18 

Silver  .  .  . 18 

Mercury .  .18 

Lead 19 

Method  of  Analysis     .         .         .         .         .         .         .         .21 

Discussion  . .  21 

HYDROGEN  SULPHIDE  GROUP 23 

DIVISION  A  .  .  23 

Mercury       .    '     .         .         .         .         .         .        ;.         .         .23 

Copper  .  .  .  »  .  .  ...  .  .  24 

Cadmium  .  .  , .25 

Bismuth .  26 

Method  of  Analysis  .  »  • 27 

Discussion 28 

DIVISION  B  .  .  . 30 

Arsenic 30 

Antimony 34 

Tin  36 


VI 


CONTENTS 


PAGE 
HYDROGEN  SULPHIDE  GROUP  (continued) 

PRECIPITATION  AND  SEPARATION  OF  DIVISIONS  A  AND  B  .  38 

Discussion 40 

DIVISION  B  .  .  .  .  .  /  .  .  .44 

First  Method  of  Analysis  .  .  .  „  .  .  44 

Discussion  .  .  .  .  .  .  .  .  .  45 

Second  Method  of  Analysis 48 

Discussion  ..........  50 

AMMONIUM  SULPHIDE  GROUP 53 

Nickel  .  .  ;  53 

Cobalt  .  . ,'  54 

Iron     .         .         .         .          .         .         .         .  '       .         .         .       55 

Manganesa  ......  .^  ..  58 

Zinc  .  ...  .  .  .  * 59 

Aluminum       .........  60 

Chromium  .          .          .          .          „  .         .  .60 

Method  of  Analysis          ,         .         .         .         .         *         .62 

Discussion  ..........  66 

AMMONIUM  CARBONATE  GROUP  AND  MAGNESIUM      .         .         .  71 

Barium         .          .          .          .         .          .         .         ...       71 

Strontium        .          .         .          .         .         .         ...  72 

Calcium       .         .         v         .         .         .         .         i;        -.         . '     73 

Magnesium     .         .         ....         .         .         .         .  73 

Method  of  Analysis  .  .  "..".-.>  .  .'74 
Discussion 76 

AMMONIA,  SODIUM,  POTASSIUM .79 

Ammonia .^^  .  .  79 

Sodium        ,         .         .         .         .  /  .   t         .-v  'lfi*T       .         .       79 

Potassium        ....         .....         .         .  79 

Detection     .         .         .         .         .         .         .         .  .80 

Discussion  81 


CHLORATES 
CARBONATES 
SULPHIDES 
SULPHITES    . 
CYANIDES 


PART    III 
THE  ACIDS 


83 
85 
86 

87 
88 


CONTENTS  vii 

PAGE 

PHOSPHATES          . 90 

SULPHATES        ...........       91 

CHROMATES  .  92 

BORATES  ...         .         . 94 

OXALATES    .        .        .        .        .        .         .         .         .       ...        .  96 

TARTRATES  AND  ACETATES  (ORGANIC  MATTER)  .         .  .97 

TARTRATES $  97 

ACETATES          .         .         .         .         .         .         .         .         ...         .98 

IODIDES         .         .         .    '     .         .         .         .         .         .         .         .  99 

BROMIDES .         .         .   •     .         •.         .     100 

FERROCYANIDES    .         .         .  .         v      •     »•         •         •          101 

FERRICYANIDES 103 

CHLORIDES _^       .  104 

NITRATES  106 


PART   IV 
SYSTEMATIC  ANALYSIS  OF  A   SOLID  SUBSTANCE 

I.  PREPARATION  OF  SOLUTIONS  FOR  THE  DETECTION  OF  THE  BASES  109 

a.  THE  SUBSTANCE  is  NOT  A  METAL          ;         .--..,..  109 

Water  Solution    .         .         .         .    '     .    -   4fc     •         •         •  HO 

Hydrochloric  Acid  Solution     .          .         .               |r  .         .  Ill 

Nitric  Acid  Solution    .         ..     *.         .         -V-     / '  •         •  H2 

Aqua  Regia  Solution        .         .-.         ..         .         .  112 
Residue  Insoluble  in  Water  and  Acids         .         .         .         .112 

b.  THE  SUBSTANCE  is  A  METAL  OR  AN  ALLOY          .         .  117 
II.  PREPARATION  OF  SOLUTIONS  FOR  THE  DETECTION  OF  THE  ACIDS  117 

III.  INTERPRETATION  OF  RESULTS                            .         .  122 


APPENDIX 

LIST  OF  APPARATUS    .         .         .         .         .         .         . *  -     .  .          125 

LIST  OF  REAGENTS !'•<*"         •  *     ^^ 

TABLE  OF  ATOMIC  WEIGHTS  OF  THE  ELEMENTS       .  .         134 

THE  PERIODIC  SYSTEM    .         . 135 

ABBREVIATIONS  OF  THE  TITLES  OF  JOURNALS           .         .  .          136 

TABLE  OF  METRIC  MEASURES  WITH  ENGLISH  EQUIVALENTS  .     137 

CONVERSION  FACTORS          .         .         .         .         .         .         .  .         138 

INDEX  139 


QUALITATIVE  ANALYSIS 
PART  I 


INTRODUCTION 

Qualitative  Chemical  Analysis  deals  with  the  operations  and 
methods  that  are  employed  to  ascertain  what  chemical  elements 
or  simple  chemical  compounds  are  present  in  more  complex 
substances  or  mixtures. 

Quantitative  Chemical  Analysis  treats  of  the  methods  that  are 
employed  to  determine  how  much  of  each  element  or  of  each 
simpler  compound  is  present  in  the  more  complex  substance  or 
mixture. 

It  is  apparent  that  before  the  chemist  can  proceed  with  the 
determination  of  the  amount  of  each  of  the  elements  present  in 
a  substance  he  must  know  what  elements  the  substance  con- 
tains. For  this  reason  the  qualitative  analysis  of  the  material 
in  hand  must  precede  the  quantitative  analysis. 

Qualitative  analysis  may  be  said  to  consist  chiefly  of  the 
study  of  the  solubilities  of  the  elements  and  their  compounds. 
Thus  in  the  systematic  qualitative  analysis  of  a  chemical  sub- 
stance it  is  first  brought  into  solution,  and  there  is  then  added 
to  this  solution  a  substance  of  such  a  nature  as  will  cause  com- 
pounds of  certain  of  the  elements,  if  the  latter  are  present,  to 
separate  in  a  solid  form,  i.e.,  to  be  precipitated.  Other  elements 
that  are  present  will  remain  in  solution,  and  they  are  then  sepa- 
rated from  the  precipitate  by  filtration  through  paper.  To  the 
clear  liquid  which  passes  through  the  filter,  and  which  is  termed 
the  filtrate,  another  chemical  substance  of  known  nature  is  added, 

1 


2  QUALITATIVE  ANALYSIS 

and  a  second  group  of  elements  is  precipitated.  This  precipi- 
tate is  also  separated  by  filtration,  and  a  third  group  of  elements 
is  precipitated  from  the  nitrate,  and  so  on.  The  chemicals  of 
known  nature  that  are  used  to  cause  the  various  separations  are 
termed  reagents.  When  these  elements  have  thus  been  divided 
into  groups  the  members  of  each  group  are  separated  one  from 
the  other  by  suitable  chemical  treatment,  and  careful  tests  are 
then  made  to  ascertain  which  elements  are  present  and  which 
are  absent. 

The  subject  of  qualitative  analysis  has  become  so  broad  that 
it  is  now  subdivided  into  various  branches,  such  as  the  analysis 
of  inorganic  substances,  the  analysis  of  organic  substances,  gas 
analysis,  spectroscopic  analysis,  etc. 

The  branch  of  qualitative  analysis  treated  in  this  manual 
comprises  the  detection  of  about  thirty  of  the  more  common 
elements  and  their  compounds. 

Before  the  chemist  can  ascertain  by  analysis  what  elements  are 
present  in  a  compound  or  mixture,  he  must  first  familiarize  him- 
self with  the  characteristic  properties  and  the  chemical  behavior 
of  these  different  elements.  This  information  may  be  imparted 
by  the  teacher  to  the  student  in  a  variety  of  ways.  The  proper- 
ties of  the  elements  and  their  compounds  may  be  stated  outright 
to  the  pupil,  who  then  begins  at  once  with  the  actual  analysis 
of  chemical  mixtures.  Such  a  method  makes  it  possible  to  give 
the  student  a  slight  knowledge  of  the  subject  in  a  comparatively 
short  period  of  time,  but  it  should  be  adopted  only  when  the 
time  available  for  the  work  is  very  limited,  for  the  student  will 
be  obliged  to  follow  mechanically  the  directions  that  he  receives 
and  will  fail  to  understand  the  various  steps  involved  in  the 
analysis.  It  is  unnecessary  to  emphasize  the  importance  of  this 
understanding  of  each  and  every  part  of  the  work.  Without  it 
no  one  may  hope  to  become  a  successful  chemist. 

The  other  extreme  in  the  teaching  of  the  subject  consists  in 
allowing  the  student  to  ascertain  for  himself  the  characteristic 


INTRODUCTION  3 

properties  of  the  various  elements  and  then  to  devise  his  own 
plan  for  the  qualitative  analysis  of  mixtures.  The  faults  of 
this  procedure  are  as  great  as  those  of  the  first,  for  it  is  impos- 
sible for  the  pupil  to  study  exhaustively  each  method  of  separa- 
tion ;  and,  furthermore,  he  loses  that  aid  which  is  to  be  derived 
from  the  accumulated  experience  of  the  many  chemists  who  have 
preceded  him. 

A  system  of  instruction  lying  between  the  two  extremes 
above  described  is  .therefore  to  be  preferred,  and  where  time 
will  permit,  the  authors  would  recommend  the  adoption  of  the 
following  plan : 

The  student,  using  solutions  of  the  compounds  of  the  metals 
taken  in  alphabetical  or  other  chance  order,  studies  the  action 
upon  these  solutions  of  (1)  potassium  hydroxide,  (2)  ammo- 
nium hydroxide,  (3)  sodium  carbonate,  (4)  hydrogen  sulphide, 
(5)  ammonium  sulphide,  (6)  hydrochloric  acid,  and  (7)  sulphuric 
acid.  He  next  classifies  the  elements  into  groups,  basing  the 
classification  upon  his  own  experimental  work.  The  teacher 
then  explains  the  reasons  for  the  division  of  the  elements  into 
groups  as  followed  in  the  text-book,  and  the  student  proceeds 
to  study  in  the  laboratory  these  groups  and  the  separation  from 
one  another  of  the  elements  in  each  group,  this  work  being 
accompanied  by  lectures  and  recitations. 

REACTIONS 

The  chemical  change  that  takes  place  between  two  or  more  sub- 
stances when  they  are  brought  together  is  termed  a  reaction,  and 
for  the  sake  of  brevity  it  is  expressed  by  the  chemist  by  means 
of  an  equation  in  which  the  different  elements  are  represented 
by  symbols.  Thus,  if  ammonium  hydroxide  is  added  to  a  solu- 
tion of  ferric  chloride,  a  reaction  takes  place  and  there  is  formed 
ferric  hydroxide,  which  is  insoluble  in  the  liquid  that  is  present, 
and  therefore  separates  as  a  precipitate.  The  other  product  of 


4  QUALITATIVE  ANALYSIS 

the  reaction  is  ammonium  chloride,  which  remains  dissolved  in 
the  water.  This  reaction  is  expressed  by  the  equation, 

FeCl3  +  3  NH4OH  =  Fe(OH)3  +  3  NH4C1. 

In  many  cases  of  chemical  change  the  reaction  is  not  as  com- 
plete and  definite  in  character  as  the  equation  might  lead  the  stu- 
dent to  suppose.  Thus,  if  a  solution  of  zinc  chloride  is  treated 
with  hydrogen  sulphide,  some  of  the  zinc  will  be  precipitated 
as  zinc  sulphide  and  some  hydrochloric  acid  will  be  formed,  the 
substances  tending  to  react  as  in  the  following  equation  : 

ZnCl2  +  H2S  =  ZnS  +  2  HC1. 

The  free  hydrochloric  acid  that  is  formed  has,  however,  a 
solvent  action  upon  the  zinc  sulphide,  tending  to  transform  it 
to  zinc  chloride  with  the  liberation  of  hydrogen  sulphide,  thus : 

ZnS  +  2  HC1  =  ZnCl2  +  H2S. 

It  is  therefore  evident  that  complete  precipitation  of  the  zinc 
from  a  neutral  solution  of  zinc  chloride  cannot  be  effected  by 
means  of  hydrogen  sulphide.  There  is  a  tendency  toward  the 
precipitation  of  zinc  sulphide,  on  the  one  hand,  and  the  decom- 
position of  the  sulphide  by  the  hydrochloric  acid,  on  the  other. 
A  reaction  of  this  nature  may  be  represented  as  follows : 

ZnCl2  +  H2S  ±z^  ZnS  +  2  HC1, 

the  two  arrows  showing  that  the  reaction  may  proceed  in  both 
directions.  Under  suitable  conditions,  however,  the  reaction 
may  be  forced  to  proceed  in  but  one  direction ;  that  is,  the  com- 
plete precipitation  of  the  zinc  may  be  effected.  This  results 
when  an  alkali  acetate  is  added  to  a  solution  of  zinc  chloride 
before  the  treatment  with  hydrogen  sulphide.  This  acetate  — 
sodium  acetate  is  usually  employed  —  reacts  with  the  hydro- 
chloric acid  as  fast  as  the  latter  is  set  free,  forming  sodium 
chloride  and  free  acetic  acid.  Zinc  sulphide  is  insoluble  in  the 
free  acetic  acid,  and  the  complete  precipitation  of  that  substance 


INTRODUCTION  5 

will  therefore  result.  Under  these  circumstances  the  reaction 
will  proceed  in  but  one  direction,  and  the  sign  of  equality  then 
correctly  represents  the  chemical  change  that  takes  place.  In 
chemical  analysis  such  reactions  as  proceed  to  a  definite  end 
are  almost  invariably  used.  If  the  work  is  carried  on  under 
the  conditions  which  are  necessary  for  bringing  about  definite 
reactions,  the  student  may  dismiss  from  his  mind  the  possibility 
of  reversal. 

The  effect  of  temperature  upon  the  course  of  a  reaction  is 
often  quite  marked.  Not  only  is  the  velocity  of  the  reaction 
usually  increased  by  heating  the  reacting  substances,  but  even 
the  very  nature  of  the  product  is  frequently  dependent  upon 
the  temperature  at  which  the  reaction  takes  place.  Thus,  if  a 
solution  of  copper  sulphate  is  treated  with  potassium  hydroxide, 
both  solutions  being  cold,  light  blue  cupric  hydroxide,  Cu(OH)2, 
is  precipitated.  This  precipitate  is  difficult  to  filter  and  wash. 
If,  however,  the  two  chemicals  are  brought  together  and  then 
heated  for  some  time  nearly  to  boiling,  the  blue  cupric  hydroxide 
changes  to  black  cupric  oxide,  which  can  easily  be  filtered. 

The  course  of  a  reaction  is  also  often  markedly  affected  by  the 
concentration  of  the  solutions  of  the  reacting  substances.  Thus, 
if  a  mixture  of  the  sulphides  of  nickel,  cobalt,  and  manganese 
be  treated  with  cold  dilute  hydrochloric  acid,  the  sulphide  of 
manganese  will  be  dissolved,  while  the  sulphides  of  nickel  and 
cobalt  will  scarcely  be  attacked.  If,  however,  a  more  concen- 
trated hydrochloric  acid  be  employed,  some  of  the  sulphides  of 
nickel  and  cobalt  will  be  dissolved. 

The  instances  above  cited  are  simple  illustrations  of  the 
broad  rule  that  the  course  of  every  reaction  is  dependent  upon 
the  prevailing  conditions.  In  general  it  is  necessary  for  success 
in  analytical  work  that  these  conditions  be  known,  so  that  if 
they  do  not  already  prevail  before  a  certain  operation  is  per- 
formed they  may  artificially  be  created.  In  some  cases  the 
conditions  may  be  varied  within  wide  limits  without  altering 


6  QUALITATIVE  ANALYSIS 

the  course  of  the  reaction  ;  in  others  any  variation  of  condition 
is  of  pronounced  effect. 

In  this  manual,  wherever  the  separation  of  elements  from 
one  another  is  involved,  the  conditions  that  must  prevail  are 
discussed.  In  the  preliminary  reactions,  on  the  other  hand,  a 
statement  that  the  interaction  of  two  chemicals  produces  a 
precipitate  of  definite  composition  must  always  be  interpreted 
to  mean  that  this  substance  will  be  formed  under  certain  con- 
ditions, which,  for  the  purposes  of  this  manual,  it  is  not  deemed 
necessary  to  define  more  closely. 

In  the  reactions  of  sodium  or  potassium  hydroxide  with  solu- 
tions of  salts  of  the  metallic  elements  it  may  be  assumed  that, 
when  the  conditions  are  so  chosen  as  to  prevent  the  formation  of 
a  basic  salt,  the  precipitate  that  is  at  first  produced  is  in  every 
case  the  normal  hydroxide.  A  few  of  these  hydroxides  break 
down  almost  immediately  into  water  and  the  oxide,  so  that  the 
precipitate  may  be  said  to  be  the  oxide.  Others  suffer  dehy- 
dration more  slowly  or  only  under  certain  conditions,  while  by 
far  the  greater  number  do  not  undergo  this  change  as  long  as 
they  are  moist.  This  adherent  water,  however,  must  be  removed 
before  the  precipitate  can  be  analyzed,  and  in  this  operation  we 
meet  new  conditions  which  are  favorable  to  dehydration.  It  is 
legitimate,  therefore,  to  represent  these  precipitates  by  the  for- 
mulas of  the  normal  hydroxides  unless  dehydration  is  known  to 
take  place  rapidly  under  ordinary  conditions. 

When  these  hydroxides  dissolve  readily  in  potassium  or  sodium 
hydroxide  it  is  probable  that  the  salts  that  are  formed  in  solu- 
tion are  derived  from  the  normal  hydroxide  by  replacement  of 
the  hydrogen  of  the  hydroxyl  groups  by  the  alkali  metal.  Thus, 
when  antimony  hydroxide  is  treated  with  sodium  hydroxide,  we 
may  assume  that  the  following  reaction  takes  place  : 

OH     HONa  ONa 


\OH     HONa          \ONa 


INTRODUCTION  7 

When  an  attempt  is  made  to  prepare  this  salt  in  a  solid  form, 
however,  it  suffers  hydrolysis ;  that  is,  it  is  more  or  less  decom- 
posed by  water,  and  instead  of  Sb(ONa)3  there  is  obtained 
SbO(ONa)  or  Sb2O3,  according  to  the  extent  to  which  hydrolysis 
has  taken  place.  Since  it  is  probable  that  these  salts,  as  long 
as  they  remain  in  solution,  are  derived  from  the  normal  hydrox- 
ides, they  will  be  so  represented  in  this  manual. 

EQUATIONS 

As  has  previously  been  stated,  an  equation  is  a  brief  form  of 
expression  of  the  chemical  change  that  takes  place  when  two 
or  more  reacting  substances  are  brought  together.  An  equation 
cannot  be  written  unless  the  reacting  substances  and  all  of  the 
products  of  the  reaction  are  known.  For  this  reason  the  first 
step  in  thus  expressing  a  chemical  change  consists  in  writing 
on  the  left  of  the  sign  of  equality  the  formulas  of  those  sub- 
stances which  react  upon  one  another,  and  upon  the  right  of 
the  sign  the  formulas  of  all  of  the  products  of  this  reaction. 
When  this  has  been  done  all  that  is  necessary  to  complete  the 
equation  is  to  insert  before  the  different  formulas  such  num- 
bers as  will  represent  the  relative  amounts  of  the  substances 
entering  into  the  reaction.  A  case  of  simple  transposition 
presents  no  difficulty  to  the  student  who  has  become  familiar 
with  the  formulas  of  the  acids  and  their  salts.  For  example, 
when  barium  chloride  and  sodium  sulphate  interact  we  know 
from  investigation  that  barium  sulphate  and  sodium  chloride 
are  formed.  In  representing  this  by  means  of  an  equation 
it  is  at  once  apparent  that  if  the  barium  and  the  sodium 
simply  change  places  in  the  two  salts,  one  molecule  of  barium 
chloride  will  act  upon  one  molecule  of  sodium  sulphate,  form- 
ing one  molecule  of  barium  sulphate  and  two  of  sodium 
chloride,  thus: 

BaCl2  +  Na2SO4  =  BaSO4  +  2  NaCl. 


8  QUALITATIVE  ANALYSIS 

When,  however,  a  reaction  involves  oxidation  or  reduction, 
the  insertion  of  the  numbers  directly  representing  the  relative 
amounts  of  the  reacting  substances  and  of  the  products  becomes 
more  difficult.  Thus,  when  concentrated  nitric  acid  acts  upon 
metallic  copper,  investigation  has  shown  that  the  products  are 
copper  nitrate,  nitric  oxide,  and  water : 

Cu  +  HN03-   ->Cu(N03)2  +  NO  +  H20. 

It  would  be  possible  by  repeatedly  trying  different  numbers  to 
chance  upon  those  which  correctly  express  the  relative  amounts 
of  copper  and  nitric  acid  that  react  upon  each  other,  and  the 
amounts  of  the  different  products.  The  waste  of  time  that 
such  a  procedure  would  involve  may  be  avoided  in  a  very 
simple  manner.  Thus  in  the  reaction  just  given  we  may  assume 
that  when  copper  is  acted  upon  by  nitric  acid  it  is  first  oxidized 
to  copper  oxide.  To  change  one  atom  of  copper  to  copper 
oxide,  one  atom  of  oxygen  is  necessary: 

Cu  +  O  -  CuO. 

This  oxygen  is  furnished  by  the  nitric  acid.  It  is  known  that, 
when  nitric  acid  acts  as  an  oxidizing  agent  under  the  conditions 
prevailing  in  this  experiment,  two  molecules  of  the  acid  furnish 
three  atoms  of  oxygen  that  are  available  for  the  oxidation  of 
the  copper.  The  other  products  of  this  decomposition  of  the 
nitric  acid  are  water  and  nitric  oxide  : 

2  HNO3  =  H2O  +  2  NO  +  3  O. 

It  is  therefore  evident  that  two  molecules  of  nitric  acid  can 
oxidize  three  atoms  of  Cu  to  CuO : 

3  Cu  +  2  HNO3  =  H2O  +  2  NO  +  3  CuO. 

But  copper  nitrate  and  not  copper  oxide  is  formed  in  the  reaction. 
This  means  that  more  nitric  acid  reacts  on  the  copper  oxide, 
which  we  have  assumed  is  formed  in  the  beginning,  changing  it 
to  copper  nitrate.  To  transform  the  three  molecules  of  CuO  to 


INTRODUCTION  9 

the  nitrate,  there  will  be  needed  six  more  molecules  of  nitric 

acid : 

3  CuO  +  6  HN03  =  3  Cu(NO3)2  +  3  H2O. 

Hence  the  total  amount  of  nitric  acid  required  by  the  three 
atoms  of  copper  will  be  two  molecules  for  the  oxidation  to  the 
copper  oxide,  and  six  more  for  the  transformation  of  the  oxide 
to  the  nitrate,  or  eight  in  all.  We  are  now  ready  to  complete 
the  equation,  and  it  becomes 

3  Cu  +  8  HNO3  =  3  Cu(NO3)2  +  2  NO  +  4  H2O. 

In  solving  such  equations  it  is  frequently  of  advantage  to 
consider  the  oxygen  acids  as  made  up  of  water  and  the  acid 
anhydride,  and  the  salts  of  such  acids  as  made  up  of  an  oxide 
of  a  metal  and  the  acid  anhydride.  Thus : 

Sulphuric  acid  H2SO4  H2O-SO3 

Nitric  acid  HNO3  H2O-N2O5 

Chloric  acid  HC1O3  H2O-C12O5 

Sulphurous  acid  H2SO3  H2O-SO2 

Ferrous  sulphate  FeSO4  FeO-SO3 

Ferric  sulphate  Fe2(SO4)3  Fe2O3-3  SO3 

Copper  nitrate  Cu(NO3)2  CuO-N2O5 

Potassium  chlorate  KC1O3  K2O-C12O5 

Sodium  sulphite  Na2SO8  Na2O-SO2 

To  illustrate  the  employment  of  such  formulas  let  us  consider 
the  action  of  nitric  acid  on  mercurous  nitrate.  The  products  of 
this  reaction  are  known  to  be  mercuric  nitrate,  nitric  oxide,  and 
water,  HgNQ3  +  HNO3  — >  Hg(NO3)2  +  NO  +  H2O, 
or,  representing  the  acid  and  salts  in  the  manner  just  described, 
Hg2ON2O5  +  H2ON205  — >  HgON205  +  NO  +  H2O. 

It  has  already  been  stated  that  when  nitric  acid  breaks  down 
with  the  formation  of  nitric  oxide  and  water,  three  atoms  of 
oxygen  are  available  for  oxidation, 

H2ON205  =  H20  +  2  NO  +  3  O. 


10  QUALITATIVE  ANALYSIS 

One  atom  of  oxygen  can  change  one  Hg2O  to  Hg2O2  or  2  HgO. 
Hence  the  3  O  from  the  nitric  acid  will  oxidize  3  Hg2O  to  6  HgO. 
Inserting  these  figures  in  the  equation  we  have 

8(Hg,0-N,08)  +  l(H,0-N,05)  —  >6(HgON2O5)+  2NO+H2O. 

The  six  molecules  of  mercuric  nitrate  contain  6  N2O5.  Only 
3  N2O5  are  furnished  by  the  mercurous  nitrate.  Hence,  to 
obtain  the  remainder  of  the  N2O5,  three  more  (H2O-N2O5)  are 
necessary  in  addition  to  the  1(H2O-N2O6)  that  was  needed  for 
the  oxidation.  This  makes  4(H2O-N2O5)  in  all.  The  water 
of  the  acid  is  set  free,  appearing  as  4  H2O.  The  completed 
equation  is 

3(Hg20-N205)  +  4(H2ON206)  =  6(HgO-N2O6)  +  2  NO  +  4  H2O, 
or,  written  in  the  usual  manner, 

6  HgN03  +  8  HN03  =  6  Hg(NO3)2  +  2  NO  +  4  H2O. 

These  numbers  contain  a  common  factor,  2.  Dividing  by  this 
we  have 

3  HgN03  +  4  HN03  =  3  Hg(NO3)2  +  NO  4-  2  H2O. 

Another  instructive  illustration  of  this  method  is  furnished  by 
the  reaction  taking  place  when  chromium  oxide  is  fused  with 
sodium  nitrate  and  sodium  carbonate. 

Cr2O3  +  NaNO3  4-  Na2CO3  -  >  Na2CrO4  4-  CO2  +  NO, 
or 

Cr2O3  4-  Na2O-N2O5  +  Na2O-CO2  -  >  Na2O-CrO3  4-  CO2  +  NO. 

The  Cr2O3  is  oxidized  to  2  CrO3,  the  oxygen  being  furnished 
by  the  N2O5  of  the  sodium  nitrate.  l(Na2O-N2O5)  furnishes  3  O. 
To  oxidize  1  Cr2O3  to  2  CrO3  requires  3  O.  Therefore, 

1  Cr203  +  l(Na2O-N205)  +  Na2O-CO2  -  > 


2(Na20-003)  +  CO2  +  2  NO. 

CrO3  is  an  acid  oxide,  and  unites  with  the  alkali  base,  Na2O, 
to  form  sodium  chromate.     The  2(Na2O-CrO3)  contains  2  Na2O. 


INTRODUCTION  11 

Only  one  of  these  is  furnished  by  the  l(Na2ON2O5).  The 
other  comes  from  the  sodium  carbonate  which  is  introduced 
for  just  this  purpose.  When  the  Na2O  of  the  Na2OCO2 
unites  with  CrO3,  the  CO2  of  the  carbonate  is  set  free.  The 
equation  now  stands : 

Cr203  +  2  NaN03  +  Na2CO3  =  2  Na2CrO4  +  CO2  +  2  NO. 

This  method  of  solving  equations  involving  oxidation  and 
reduction  is  of  general  applicability,  and  it  will  be  employed 
continually  throughout  the  work.  It  is  of  marked  usefulness  in 
quantitative  analysis  as  well. 

PRECIPITATION 

In  the  usual  methods  of  qualitative  analysis  an  element  is 
generally  recognized  by  causing  it  to  unite  with  other  elements 
to  form  compounds  of  known  properties.  These  compounds 
are  usually  insoluble  in  the  liquid  that  is  present,  and  they 
therefore  separate  as  precipitates. 

It  should  be  borne  in  mind,  however,  that  strictly  speaking 
no  precipitation  is  ever  entirely  complete.  The  precipitated 
solid  dissolves  in  the  liquid  that  is  present  until  the  latter  is 
saturated.  In  most  of  the  reactions  that  are  employed  by  the 
analytical  chemist  this  solubility  is  very  slight,  for  the  condi- 
tions prevailing  at  the  time  of  the  precipitation  are  so  chosen 
as  to  reduce  it  to  a  minimum.  For  example,  it  may  be  lessened 
(1)  by  varying  the  temperature,  (2)  by  adding  a  liquid  in  which 
the  precipitate  is  less  soluble  than  in  the  original  solution,  or 
(3)  by  removing  the  substance  in  which  the  precipitate  is 
soluble;  (4)  the  solubility  of  a  precipitate  is  also  sometimes 
decreased  by  adding  to  the  mixture  a  substance  that  contains 
an  element  or  group  that  is  also  present  in  the  precipitate. 

The  effect  of  temperature  upon  the  completeness  of  precipi- 
tation is  strikingly  shown  by  the  action  of  hydrochloric  acid 


12  QUALITATIVE  ANALYSIS 

upon  a  concentrated  solution  of  a  lead  salt.  If  the  acid  is 
added  to  a  cold  solution  of  the  lead  compound,  almost  all  of  the 
lead  will  be  precipitated  as  lead  chloride,  for  this  substance  is 
but  slightly  soluble  in  cold  water.  But  if  the  solutions,  before 
mixing,  are  heated  nearly  to  boiling,  almost  no  lead  chloride 
will  appear  because  of  its  solubility  in  hot  water. 

An  illustration  of  the  reduction  of  solubility  by  the  addition 
of  a  liquid  in  which  the  precipitate  is  less  soluble  than  in  that 
originally  present  is  to  be  found  in  the  precipitation  of  lead  by 
dilute*  sulphuric  acid.  The  lead  sulphate  is  somewhat  soluble 
in  water,  but  if  alcohol  is  added  it  is  precipitated  almost 
completely. 

The  increase  in  the  completeness  of  precipitation  resulting 
from  the  removal  of  a  substance  from  solution  is  shown  in  the 
precipitation  of  aluminum  hydroxide  by  ammonium  hydroxide. 
Aluminum  hydroxide  is  appreciably  soluble  in  the  excess  of 
ammonia  that  is  present  after  precipitation.  If  this  be  removed 
by  heating  the  liquid,  more  complete  precipitation  of  the 
aluminum  hydroxide  results. 

The  decrease  in  the  solubility  of  a  precipitate  consequent 
upon  the  addition  of  a  substance  containing  an  element  or 
group  that  is  also  present  in  the  precipitate  may  be  illustrated 
by  the  precipitation  of  sulphuric  acid  by  a  solution  of  barium 
chloride.  If  there  is  added  just  enough  barium  chloride  to  pre- 
cipitate the  sulphuric  acid,  some  of  the  barium  sulphate  will 
remain  in  solution  ;  but  if  more  of  the  barium  chloride,  which 
contains  the  element  barium  in  common  with  the  precipitated 
barium  sulphate,  is  added,  the  solubility  of  the  barium  sulphate 
in  the  liquid  will  be  lessened  and  the  precipitation  will  conse- 
quently be  more  complete.  It  is  for  this  reason  that  an  excess 
of  the  precipitant  should  usually  be  added,  and  the  excess  should 
be  greater,  the  greater  the  solubility  of  the  precipitate. 

Certain  amorphous  substances,  termed  colloids,  tend  to 
dissolve  in  water  and  form  pseudo-solutions.  Aluminum 


INTRODUCTION  13 

hydroxide,  ferric  hydroxide,  and  arsenious  sulphide  are  familiar 
examples  of  such  bodies.  These  colloids  are  precipitated  from 
their  pseudo-solutions  by  the  addition  of  a  salt  solution  or  an 
acid.  Thus,  if  a  solution  of  arsenious  acid  in  water  be  treated 
with  hydrogen  sulphide,  almost  no  precipitate  appears,  but  there 
is  formed  chiefly  a  cloudy  liquid.  Upon  the  addition  of  an  acid 
or  a  solution  of  a  salt  to  this  liquid  the  yellow  sulphide  of  arsenic 
will  be  precipitated.  Colloids  become  denser  and  less  soluble  at 
high  temperatures,  and  it  is  therefore  well  to  precipitate  them 
from  hot  solutions. 

When  a  liquid  reagent  is  added  to  the  solution  under  analysis 
it  should  be  put  in  at  first  drop  by  drop,  and  the  student  should 
carefully  observe  the  effect  of  such  addition.  The  solution 
should  not  be  shaken  as  soon  as  some  of  the  reagent  is  added, 
for  it  frequently  happens  that  characteristic  reactions,  giving 
important  information  concerning  the  nature  of  the  substances 
under  examination,  take  place  where  the  reagent  comes  in  con- 
tact with  the  other  liquid.  Any  change  in  the  color  of  the 
solution,  and  the  odor  or  color  of  any  gas  that  may  be  evolved, 
should  carefully  be  noted.  The  reagent  may  then  be  added 
in  an  amount  sufficient  to  react  with  all  of  the  substance  in 
solution.  It  is  usually  impossible  to  ascertain,  except  by  deli- 
cate chemical  tests,  just  when  this  amount  has  been  introduced. 
Hence  an  excess  of  the  reagent  (but  only  a  slight  excess)  is 
ordinarily  employed.  To  make  sure  that  such  an  excess  has 
been  added  it  is  well  to  test  the  filtrate  with  an  additional  drop 
or  two  of  the  reagent.  If  further  precipitation  results  on  this 
addition,  more  of  the  reagent  must  be  added  to  the  entire 
filtrate,  and  the  filtration  must  be  repeated. 

To  ascertain  whether  a  reagent,  if  employed  in  excess,  will 
redissolve  a  precipitate  that  it  has  produced,  complete  precipita- 
tion is  first  effected  and  then  more  of  the  reagent  is  added  and 
the  mixture  is  shaken  or  stirred.  If  no  action  is  visible,  the  vessel 
containing  the  liquid  is  then  carefully  heated  over  a  Bunsen  flame. 


14  QUALITATIVE  ANALYSIS 

Some  precipitates  dissolve  more  readily  in  an  excess  of  the 
reagent  if  such  excess  is  added  as  soon  as  the  precipitate  is 
formed.  It  is  therefore  well  to  supplement  the  procedure 
described  above  by  placing  in  a  test  tube  a  few  drops  of  the 
solution  to  be  precipitated  and  adding  at  once  a  large  excess  of 
the  reagent. 

When  it  is  desired  to  heat  more  than  a  very  small  quantity 
of  a  liquid  in  a  test  tube  the  flame  should  be  applied  to  the  tube 
somewhat  below  the  surface  of  the  liquid.  If  the  heat  is 
applied  at  the  lower  end  of  the  tube,  bubbles  of  steam  will  form 
at  the  bottom,  and,  rising,  will  force  the  liquid  before  them  and 
may  drive  it  out  of  the  mouth  of  the  tube.  The  flame  should 
not  be  allowed  to  strike  the  glass  above  the  surface  of  the 
liquid,  for  the  glass  might  then  become  so  hot  as  to  break  when 
the  liquid  comes  in  contact  with  it.  Wooden  test-tube  holders 
are  sometimes  used  for  holding  the  tube  in  the  flame,  but  a  less 
expensive  and  more  convenient  device  consists  of  a  piece  of 
stiff  paper  or  a  strip  of  cloth  that  is  wrapped  around  the 
upper  part  of  the  tube.  The  projecting  ends  of  the  strip  are 
held  between  the  thumb  and  finger. 

FILTRATION 

To  separate  a  precipitate  from  the  liquid  in  which  it  is  sus- 
pended, it  is  brought  upon  a  filter  paper  which  is  supported  in  a 
glass  funnel.  The  student  is  supplied  with  filter  paper  already 
cut  in  circular  form.  To  prepare  one  of  these  disks  for  use  it 
is  folded  twice,  so  that  it  forms  a  quadrant.  This  is  opened  so 
that  three  layers  of  the  paper  are  upon  one  side  and  one  layer 
upon  the  other,  and  it  is  then  inserted  into  a  dry  funnel.  After 
wetting  the  paper  with  distilled  water  from  the  wash  bottle  it 
should  be  fitted  snugly  to  the  glass  by  gentle  pressure  with  the 
finger,  and  the  water  remaining  on  the  filter  should  be  allowed 
to  run  through  and  should  not  be  poured  out.  The  funnel  is 


INTRODUCTION  15 

then  placed  over  the  vessel  that  is  to  receive  the  filtrate.  If  the 
funnel  is  of  proper  shape  and  the  paper  in  correct  position, 
the  stem  of  the  funnel  will  fill  with  liquid  during  filtration, 
and  the  process  will  thus  be  greatly  hastened.  The  filter 
should  never  be  filled  to  the  top,  for  then  the  precipitate  could 
not  be  washed  with  a  jet  of  water  without  danger  of  carrying 
some  of  it  over  the  edge  of  the  paper  and  down  the  walls  of 
the  funnel  into  the  filtrate. 

Many  substances  when  first  precipitated  are  so  finely  divided 
that  they  will  pass  through  the  pores  of  the  filter  paper.  If, 
before  filtering,  the  precipitate  is  heated  with  the  liquid  and  is 
allowed  to  stand  hot  for  some  time,  the  particles  of  the  precipi- 
tate become  larger,  and  the  substance  is  then  easily  retained  by 
the  paper.  A  good  illustration  of  a  precipitate  of  this  character 
is  barium  sulphate.  If  this  substance  is  precipitated  from  cold 
solutions,  it  passes  readily  through  the  pores  of  the  paper,  but 
it  is  easily  retained  by  the  paper  if  treated  in  the  manner  above 
described. 

If  the  precipitate  settles  readily  or  is  of  such  a  nature  as  to 
clog  the  pores  of  the  filter,  it  may  be  washed  by  decantation 
before  being  brought  upon  the  filter.  This  is  done  by  allowing 
the  precipitate  to  settle  and  then  carefully  pouring  off  the  liquid 
through  the  filter,  keeping  back  as  much  of  the  precipitate  as 
possible  in  the  dish  in  which  it  was  formed.  After  most  of  the 
supernatant  liquid  has  thus  been  removed,  water  is  poured  upon 
the  precipitate  and  heated  to  boiling.  The  whole  is  thoroughly 
shaken,  the  precipitate  allowed  to  settle,  and  the  liquid  again 
poured  off  through  the  filter.  The  filtrate  is  then  set  aside  for 
further  examination,  and  the  precipitate  is  washed  two  or  three 
times  more  by  decantation,  this  wash  water  now  being  thrown 
away. 

If  a  precipitate  does  not  settle  readily,  it  is  at  once  brought 
upon  the  filter,  the  liquid  allowed  to  run  through,  and  the 
precipitate  is  then  washed  with  distilled  water  with  the  aid  of 


16  QUALITATIVE  ANALYSIS 

a  wash  bottle,  the  jet  of  water  being  directed  along  the  upper 
edge  of  the  paper.  The  paper  should  be  filled  nearly  to  the 
top  with  water,  and  all  of  this  liquid  should  be  allowed  to  run 
through  before  more  water  is  added. 

Unless  otherwise  directed,  hot  water  should  always  be 
employed  in  washing  precipitates.  The  reason  for  this  is  two- 
fold. In  the  first  place  the  internal  friction  of  water  at  100°  is 
less  than  one-sixth  of  what  it  is  at  0°,  and  consequently  filtra- 
tion will  proceed  more  rapidly  when  hot  solutions  are  used. 
Moreover,  the  soluble  salts  that  are  to  be  separated  from  the 
precipitate  by  the  filtration  usually  dissolve  to  much  greater 
extent  in  hot  water  than  in  cold,  so  that  the  amount  of  washing 
necessary  to  carry  these  substances  into  the  filtrate  will  be  less 
if  the  wash  water  is  hot. 

If  a  precipitate  passes  through  the  paper  and  causes  a  turbid 
filtrate,  this  filtrate  should  again  be  poured  upon  the  filter  and 
the  operation  repeated  until  the  filtrate '  comes  through  clear. 
If  a  clear  filtrate  cannot  be  obtained  in  this  manner,  a  new  filter 
of  two  or  three  filter  papers  should  be  made  and  the  liquid 
filtered  through  this  thicker  layer ;  or  the  liquid  may  be  allowed 
to  stand  until  the  precipitate  settles,  and  the  supernatant  liquid 
then  poured  off  or  drawn  off  with  a  siphon. 

THE  SOLUTION  OF  THE  PRECIPITATE 

A  precipitate  that  has  been  collected  on  a  filter  is  sometimes 
dissolved  by  simply  pouring  the  solvent  upon  the  precipitate,  the 
funnel  first  being  placed  in  a  test  tube  or  flask  to  receive  the  solu- 
tion when  it  runs  through.  But  when  the  solvent  acts  upon  the 
paper,  or  when  the  precipitate  must  be  heated  with  the  solvent 
to  effect  solution,  the  filter  paper  should  be  removed  from  the 
funnel,  spread  upon  a  glass  plate,  and  the  precipitate  scraped 
off  with  a  horn  spatula  into  a  small  porcelain  dish.  The  solvent 
is  then  poured  directly  upon  the  precipitate  in  the  dish. 


•  INTRODUCTION  17 

If  the  amount  of  the  precipitate  is  very  small,  it  is  washed 
into  the  apex  of  the  filter,  the  paper  is  removed  from  the 
funnel,  and  the  point  containing  the  precipitate  is  torn  off  and 
placed  in  a  porcelain  dish.  The  paper  is  spread  out  with  the 
precipitate  uppermost,  and  the  solvent  is  then  added. 

' 
EVAPORATION 

If  a  liquid  is  to  be  driven  off  by  evaporation,  it  should  be 
placed  in  a  porcelain  evaporating  dish;  for  if  a  glass  vessel  is 
heated  over  a  flame,  it  may  break  as  soon  as  the  liquid  is  driven 
off  and  the  dish  becomes  dry.  A  porcelain  evaporator  may  be 
heated  directly  over  the  free  flame  without  the  use  of  wire 
gauze. 

If  the  contents  of  the  dish  gives  off  acid  fumes  or  a  strong 
odor  of  ammonia  during  the  evaporation,  it  should  be  placed  in 
the  hood. 

The  heating  of  solid  residues  to  drive  off  ammonium  salts 
should  always  be  carried  on  in  the  hood. 


PAET  ;  II 

THE  BASES 


• 

, 

HYDROCHLORIC  ACID   GROUP 
PRELIMINARY   REACTIONS 

SILVER:  Reactions  of  solutions  of  silver  salts. 

KOH  or  NaOH  precipitates  Ag2O,  brown. 

NH4OH  produces  in  neutral  solutions  a  white  precipitate,  prob- 
ably AgOH,  which  changes  immediately  to  Ag2O.  When  more 
NH4OH  is  added  the  precipitate  dissolves,  forming  salts,  such 
as  AgNO3-2NH3,  diammonia  silver  nitrate. 

Na2C03  precipitates  Ag2CO3,  wfcite.  or  a  basic,  carbonate, 
yellow  ;  soluble  in  large  excess  of  the  concentrated  reagent. 

H2S  precipitates  Ag2S,  black  :  soluble  in  hot  dilute  HNO3, 
usually  with  an  evolution  of  H2S,  although  a  partial  oxidation 
of  the  latter  to  sulphur  by  the  HNO3  may  also  take  place. 

(NH4)2S  precipitates  Ag2S. 

HC1  precipitates  AgCl,  white,  curdy,  changing  on  exposure  to  the  light 
through  various  shades  'from  lavender  to  black ;  easily  soluble  in  NH4OH, 
forming  2AgCl-3NH3.1  From  this  solution  HN03  reprecipitates  AgCl. 
AgCl  is  somewhat  soluble  in  concentrated  HC1.  - 

1^804  precipitates  from  concentrated  solutions  Ag2SO4,  white, 
crystallm^gparingly  soluble  in  water. 

MERCURY:  Reactions  of  solutions  of  mercurous  salts.. 

KOH  or  NaOH  produces  a  black  precipitate  which  is  a  mixture  2 
of  Hg2O,  HgO,  and  Hg. 

1  Jarry  :  Compt.  rend.,  124,  288  (1897). 

2  Barfoed  :  J.  prakt.  Chem.,  38,  441  (1888). 

18 


LEAP.  19 

NH4OH,  when  added  to  a  solution  of  HgNO3,  produces  a  blank 
precipitate  which  is  a  mixture  1  of  Hg  and  Hg2-N-NO3. 

Na2C03  precipitates  light  yellow  basig  carbonates,  changing  to 
gray  because  of  partial  decomposition  into  HgO,  Hg,  and  CO2. 
The  basic  carbonates  are  soluble  irl  large  excess  of  the  concen- 
trated reagent. 

H2S  produces  a  blafck  precipitate  which  is  a  mixture2  of  HgS 
and  Hg. 

(NH4)2S  produces  a  precipitate  which  is  probably  a  mixture 
of  HgS  and  Hg.  If  the  (NH4)2S  contains  an  excess  of  sulphur 
the  precipitate  consists  of  HgS. 

HC1  precipitates  HgCl,  white ;  insoluble  in  water  or  dilute  acids ; 
changed  by  NH4OH  to  a  black  mixture3  of  Hg  and  Hg-NH^Cl. 

H2S04  precipitates  Hg2SO4,  white,  crystalline,  somewhat 
soluble  in  water. 

K2Cr04  precipitates  Hg2CrO4,  red,  insoluble  in  KOH. 

SnCLj  added  in  small  quantity  to  a  dilute  solution  precipitates 
HgCl.  If  the  amount  of  SnCl2  is  increased  and  the  mixture 
warmed,  the  precipitate  is  reduced  to  black,  finely  divided 
mercury.  Under  ordinary  conditions  the  reduction  is  incom- 
plete, and  the  mixture  of  mercury  and  unreduced  HgCl  has  a 
gray  color. 

LEAD:  Reactions  of  solutions  of  lead  salts. 

KOH  or  NaOH  precipitates  basic  salts  or  Pb(OH)2  according  to 
the  conditions  under  which  precipitation  takes  place  ;  white, 
soluble  in  an  excess  of  the  reagent,  forming  salts  of  plumbous 
acid,  as  potassium  plumbite,  Pb(OK)2. 

1  Barfoed  :  J.  prakt.  Chem.,  39,  204  (1889),  has  shown  that  this  precipitate 
is  a  mixture  of   metallic   mercury  and  the  white  precipitate  that  is  formed 
when  ammonia  acts  on  mercuric  nitrate.     Hof  mann  and  Marburg  have  shown 
the  latter  to  be  H^-N-NOs  (see  note,  page  23). 

2  IIg2S  is  stable  only  below  0°.     Antony  and  Sestini :  Gaz.  chim.  UaL,  24, 
1,  193  (1894). 

3  Barfoed  :  J.  prakt.  Chem.,  39,  211  (1889). 


20  HYDROCHLORIC  ACID  GROUP 

. 

NH4OH  precipitates  basic  salts  somewhat  soluble  in  water. 

Na2C03  precipitates  a  basic  carbonate  soluble  in  large  excess 
of  the  concentrated  reagent. 

HgS  precipitates  PbS,  black  ;  insoluble  in  (NH4)2SX1  ;  oxidized  by  hot, 
fuming  HN03  to  PbS04,  white.  PbS  is  soluble  in  warm  dilute  HN03, 
forming  Pb(N03)2  and  H2S  ;  a  part  of  the  H2S  is  at  the  same  time 
usually  oxidized  to  sulphur.  From  lead  solutions  containing  a  large 
amount  o'f  hydrochloric  acid,  H2S  may  precipitate  PbCl2-  2  PbS, 
brick  red,  transformed  into  PbS  by  further  action  of  H2S, 
more  easily  when  the  solution  is  largely  diluted  ;  the  change 
is  brought  about  instantly  if  the  red  precipitate  is  treated  with 


(NH4)2S  precipitates  PbS. 

HC1  precipitates  from  not  too  dilute  solutions  PbCl2,  white,  flocculent  ; 
easily  soluble  in  hot  water,  from  which  it  crystallizes  in  long  needles 
when  the  solution  is  cooled  ;  somewhat  soluble  in  cold  water.  PbCl2  is 
less  soluble  in  dilute  HC1  than  in  water  ;  still  less-  soluble  in 
concentrated  HC1. 


precipitates  PbS04,  white  ;  almost  insoluble  in  water  (i  part  in 
about  30,000  parts  of  water)  or  in  dilute  acids.  PbS04  is  somewhat 
soluble  in  HN03  ;  readily  soluble  in  a  warm  solution  of  ammonium 
acetate  containing  a  little  free  acetic  acid.  Precipitation  of  PbSO4 
from  dilute  solutions  takes  place  more  rapidly  if  dilute  H2SO4 
or  alcohol  is  present,  the  sulphate  being  more  insoluble  in  either 
of  these  reagents  than  in  water. 

K^CrC^  precipitates  PbCr04,  yellow  ;  easily  soluble  in  NaOH,  forming 
Pb(ONa)2  ;  less  soluble  in  dilute  HN03  than  in  NaOH. 

KI  precipitates  PbI2,  yellow;  soluble  in  hot  water,  from  which  it 
crystallizes  on  cooling  in  characteristic  shining  plates. 

1  See  list  of  reagents,  page  127. 


ANALYSIS 


21 


METHOD  OF  ANALYSIS 

Place  the  neutral  or  acid  solution  in  an  Erlenmeyer  flask  and  add  HC1  a  little 
at  a  time  as  long  as  a  precipitate  is  formed.  Shake  vigorously,  filter,  and  wash 
twice  with  cold  water.  Treat  the  precipitate  as  directed  in  the  table  below, 
If  members  of  succeeding  groups  are  known  to  be  absent,  the  filtrate  may  be 
rejected.  Otherwise  the  filtrate  or  the  solution  in  which  HC1  fails  to  produce  a 
precipitate  should  be  treated  as  directed  on  page  38. 

Boil  the  precipitate  with  water  and  filter  while  hot. 


Residue :  Silver,  Mercury 

Wash  with  hot  water  and  treat  the 
precipitate  on  the  filter  with  a  little 
NH4OH. 


Residue :  Mercury 

A  black  residue 
proves  the  pres- 
ence of  mercury. 


Filtrate:  Silver 

Acidify  with 
HN03;  a  white 
precipitate  proves 
the  presence  of 
silver. 


Filtrate:  Lead 

Divide  into  two  portions. 

To  one,  add  K2Cr04 ;  a  yellow  pre- 
cipitate soluble  in  NaOH  indicates  the 
presence  of  lead. 

To  the  other  portion,  add  KI ;  a 
yellow  precipitate  soluble  in  hot  water 
and  recrystallizing  in  plates  on  cooling 
proves  the  presence  of  lead. 


DISCUSSION 

The  members  of  the  hydrochloric  acid  group,  of  metals  are 
distinguished  from  the  metals  of  the  other  groups  by  the  insol- 
ubility of  their  chlorides  in  water  or  dilute  hydrochloric  acid. 
The  distinction  is  not  altogether  a  sharp  one,  for  lead  chloride 
is  so  soluble  in  water  that  it  may  pass  partly  or,  under  favor- 
able conditions,  entirely  into  the  filtrate  from  this  group  precipi- 
tate ;  on  the  other  hand,  hydrochloric  acid  may  precipitate  the 
oxychlorides  of  bismuth  and  antimony,  members  of  the  follow- 
ing group,  but  these  precipitates  will  dissolve  if  sufficient  hydro- 
chloric acid  is  added. 

The  separation  of  lead  from  the  other  members  of  this  group 
is  based  on  the  ready  solubility  of  its  chloride  in  hot  water, 
silver  chloride  and  mercurous  chloride  being  almost  completely 


22  HYDROCHLORIC  .  ACID   GEOUP 

insoluble  in  water.  It  not  infrequently  happens  that  the  pre- 
cipitate of  mercurous  chloride  is  so  finely  divided  that  some 
of  it  passes  through  the  filter  paper,  producing  a  turbid  filtrate. 
If,  however,  more  than  traces  of  lead  are  present  in  this  filtrate, 
the  presence  of  this  small  quantity  of  mercurous  chloride  does 
not  interfere  with  the  test  for  lead,  as  lead  chromate  is  soluble 
in  sodium  hydroxide,  while  mercurous  chromate  is  not  dissolved 
by  that  reagent. 

The  separation  of  silver  from  mercury  is  based  on  the  differ- 
ent behavior  of  their  chlorides  toward  ammonium  hydroxide, 
silver  chloride  being  changed  into  the  soluble  compound 
AgCl'2NH8,  while  mercurous  *  chloride  is  changed  into  an 
insoluble-  mixture  of  Hg-NH2-Cl  and  metallic  mercury.  Yet 
.when  the  amount  of  mercurous  chloride  is  large  and  that  of 
silver  chloride  is  small,  it  is  possible  that  most  or  even  all  of 
the  silver  chloride  may  be  reduced  to  metallic  silver  by  the 
mercury  in  the  black  mixture.1  Therefore  in  case  a  considerable 
quantity  of  the  black  compound  is  formed  and  the  ammoniacal 
k  filtrate  is  found  to  be  free  from  silver,  that  element  should  be 

5ted  for  in  the  residue.  This  may  be  done  by  heating  the 
)lack  mixture  under  the  hood  until  the  mercury  and  its  com- 
pounds are  volatilized.  The  residue  may  then  be  dissolved  in 
nitric  acid  and  the  solution  tested  for  silver. 

1  Barnes  :  Chem.  News,  51,  97  (1885).     Antony  and  Turi :  Gaz.  chim.  ital, 
23,  II,  231  (1893). 


MERCURY  23 

HYDROGEN   SULPHIDE   GROUP 

(NOTE.  —  The  precipitation  of  the  HsS  group  in  the  analysis  of  a  mixture  should 
be  started  half  an  hour  before  the  student  is  ready  to  analyze  the  precipitate.  For 
directions,  see  page  38.) 

DIVISION  A 

PRELIMINARY   REACTIONS 
MERCURY  :  Reactions  of  solutions  of  mercuric  salts. 

KOH  or  NaOH  precipitates  from  cold  solutions  at  first  reddish 
brown  basic  salts,  which  change  to  yellow  HgO  when  the  alkali 
is  in  excess.     The  yellow  oxide  when  warmed  with  the  alkalis 
changes  to  red  HgO. 

NH4OH  precipitates  Hg-NH2-Cl  from  solutions  of  HgCl2.  If 
a  solution  of  Hg(NO3)2  is  used,  Hg2-N-NO3  is  precipitated.1 
Both  of  these  precipitates  are  white. 

Na^Og  precipitates  a  basic  mercuric  carbonate,  ^^jfih  br™*™  • 
soluble  in  large  excess  of  the  concentrated  reagent. 

I^S  precipitates  HgS,  black,  insoluble  in  hot  HNO%,  but  changed  by 
prolonged  action  of  that  reagent  into  Hg(N03)2-2HgS,  white,  insoluble  in 
HNOy  Both  Hg(N03)2-2HgS  and  HgS  are  changed  by  aqua  regia  into 
sulphur  and  HgCl2  which  dissolves.  HgS  is  insoluble  in  (NH4)2SX. 

If  a  current  of  I^S  is  passed  slowly  through  a  solution  of  mercuric 
chloride,  a  white  compound,  HgCl2-  2  HgS,  is  first  precipitated.  Further 
treatment  with  H2S  changes  this  into  yellow  and  then  brown  compounds 
as  the  amount  of  HgS  in  the  precipitate  increases,  until  finally  black  HgS 
is  produced.  Some  of  these  intermediate  double  compounds  are  soluble 
in  HNOy  They  are  changed  by  (NH4)2SX  into  HgS. 

(NH4)2S  precipitates  HgS. 

HC1  produces  r^precipitate. 

I^SC^  produces  no  precipitate. 

KI  precipitates  HgI2,  red ;  soluble  in  excess,  forming  HgI2-  2  KI. 

KCN  precipitates  from  concentrated  solutions  of  mercuric 
chloride  Hg(CN)2,  white ;  fairly  soluble  in  water,  and  therefore 

1  Hofmann  and  Marburg:  Ztschr.  anorg.  Chem.,  23,  131  (1900);  Ann.  Chem., 
305,  196  (1899). 


24  HYDKOGEN   SULPHIDE   GKOUP 

not  precipitated  from  'dilute  solutions ;  easily  soluble  in  KCN, 
forming  Hg(CN)2-2  KCN.  Hg(CN)2  is  not  acted  upon  by  the 
hydroxides  of  the  alkali  metals. 

SnCl2  reduces  HgCl2  to  HgCl,  white.  If  an  excess  of  SnCl2  is  added, 
the  HgCl  is  more  or  less  reduced  to  black  metallic  mercury.  If  the  super- 
natant liquid  is  poured  off  and  the  precipitate  boiled  for  some  time  with 
HCI  and  a  little  SnCl2,  the  finely  divided  mercury  collects  in  globules. 

Na2C03  fusion.  When  a  compound  of  mercury  is  ground  with 
about  six  times  its  weight  of  dry  Na2CO3  and  the  mixture  fused 
in  a  bulb  tube,  metallic  mercury  is  deposited  on  the  walls  of  the 
tube.  This  may  be  gathered  into  globules  by  rubbing  the  deposit 
with  a  glass  rod.  (The  tube  should  be  heated  gently  at  first,  and 
any  moisture  that  may  condense  on  the  walls  of  the  tube  should 
be  removed  by  a  roll  of  filter  paper  before  fusion  takes  place.) 

COPPER:  Reactions  of  solutions  of  cupric  salts. 

KOH  or  NaOH  precipitates  Cu(OH)2,  light  blue  ;  changed  by 
boiling  with  KOH  to  CuO,  black.  Cu(OH)2  is  soluble  in  a 
large  excess  of  KOH,  forming  a  blue  solution. 

If  certain  other  substances,  such  as  tartaric,  citric,  or  arsenious 
acid,  are  present,  the  addition  of  KOH  imparts  a  deep  blue  color 
to  c%pric  solutions ;  and  if  sufficient  of  one  of  the  above- 
mentioned  acids  is  present,  no  precipitation  takes  place,  even 
on  boiling. 

NH4OH  precipitates  light  blue  basic  salts;  easily  soluble  in  excess, 
forming  deep  blue  solutions  of  ammonia  cupric  salts  such  as  CuS04'4  NH3. 
KCN  decolorizes  these  blue  solutions,  the  soluble  double  salt 
cuprous  potassium  cyanide,  CuCN'KCN,  being  formed. 

Na2C03  precipitates  greenish  blue  basic  cupric  carbonate ; 
soluble  in  large  excess  of  the  concentrated  reagent. 

I^S  precipitates 1  CuS,  black ;  changed  by  hot  HN03  into  sulphur  and 
Cu(N03)2,  which  is  soluble.  CuS  is  insoluble  in  H2SO±.  Moist  CuS  is 

1  As  to  the  formation  of  CuaS  and  sulphur  in  this  reaction,  see  Thomsen  :  Ber. 
d.  chem.  Ges.,  11,  2043  (1878);  Coppock:  Chem.  News,  73,  262  (1896);  Brauner: 
Chem.  News,  74,  99  (1896);  Rossing:  Ztschr.  anorg.  Chem.,  25,  413  (1900). 


CADMIUM  25 

oxidized  to  CuS04  when  exposed  to  the  air.  CuS  is  slightly  soluble  in 
(NH4)2SX.  From  this  solution  dilute  acids  precipitate  a  yellowish  brown 
compound,  the  composition  of  which  is  not  definitely  known.1 

Contrary  to  the  statements  in  most  text-books  CuS  is  some- 
what soluble  in  Na2SI.1  It  is,  however,  insoluble  in  Na^S. 
CuS  is  soluble  in  KCN,  forming  CuCN-KCN  and  cyanogen. 

,(NH4)2S  produces  the  same  precipitate  as  H2S. 

HCI  produces  no  precipitate. 

I^SC^  produces  no  precipitate. 

KCN  precipitates  Cu(CN)2,  unstable,  decomposing  into  cyano- 
gen and  cuprous  cyanide,  CuCN.  The  latter  is  soluble  in 
KCN,  forming  CuCN-KCN. 

K4Fe(CN)6,  potassium  ferrocyanide,  precipitates  Cu2Fe(CN)6, 
reddish  brown ;  soluble  in  water  to  a  sufficient  degree  to  color 
dilute  solutions  red  without  the  formation  of  a  precipitate  ; 
undecomposed  by  acetic  acid ;  decomposed  by  alkalies. 

Iron.  If  a  piece  of  bright  iron  foil  is  placed  in  a  solution  of 
a  copper  salt,  metallic  copper  is  deposited  on  the  iron. 

CADMIUM:  Reactions  of  solutions  ofcgadmium  salts. 

KOH  or  NaOH  precipitates  Cd(OH)2,  white. 

NH4OH  precipitates  Cd(OH)2;  soluble  in  excess,  forming  salts  such  as 
Cd(N03)2-4  NH3.  If  this  ammoniacal  solution  is  treated  with  KCN, 
a  soluble  double  salt,  Cd(CN)2-2  KCN,  is  formed. 

Na2C03  precipitates  CdCO3,  or  a  basic  carbonate,  white ;  soluble 
in  a  large  excess  of  the  concentrated  reagent. 

H^S  precipitates  CdS,  varying  in  color  from  light  yellow  to  orange 
according  to  the  conditions  under  which  precipitation  tpkes  place ;  easily 
soluble  in  HCI  or  in  hot//2S04;  changed  by  hot  HNOJ  into  sulphur  and 
Cd(N03)2,  which  is  soluble.  CdS  is  insoluble  in  (NH4)2SX.  CdS  is 
insoluble  in  KCN. 

(NH4)2S  precipitates  CdS. 

HCI  produces  no  precipitate. 

1  Rossing:   Ztschr.  anorg.  Chem.,  25,  407  (1900). 


26  HYDEOGEN  SULPHIDE   GEOUP 


4  produces  no  precipitate. 
KCN  precipitates  Cd(CN)2,  white  ;  easily  soluble  in  excess, 
forming  Cd(CN)2-2  KCN. 

BISMUTH  :  Reactions  of  solutions  of  bismuth  sa/ts. 

KOH  or  NaOH  precipitates  Bi(OH)3,  white. 

NH4OH  precipitates  Bi(OH)3  or  a  basic  salt,  white;  insoluble  in  excess; 
soluble  in  dilute  acids. 

Na2C03  precipitates  bismuthyl  carbonate,  (BiO)2CO3,  or  other 
basic  carbonates,  white;  soluble  in  excess  of  the  concentrated 
reagent. 

IljS  precipitates  Bi^,  dark  brown  ;  changed  by  hot  HNOZ  into  sulphur 
and  Bi(N03)3,  which  is  soluble.  Bi2S3  is  insoluble  in  (NH4)2SX. 

(NH4)2S  precipitates  Bi2S3. 

E^O  added  in  large  quantities  to  solutions  of  bismuth  salts  that  do 
not  contain  too  much  acid,  precipitates  basic  salts,  as  BiOCl,1  (BiO)2S04, 
BiON03,  etc.  The  basic  salts  of  bismuth  are  soluble  in  dilute  inorganic  acids. 

HC1  precipitates  BiOCl  l  from  a  solution  of  Bi(N03)3  that  does  not 
contain  too  much  acid,  the  oxygen  being  furnished  by  the  water  which 
is  present.  Since  BiOCl  is  soluble  in  HC1,  the  precipitate  is  best  obtained 
by  adding  a  small  amount  of  the  bismuth  solution  to.  a  large  amount  of 
water  containing  a  little  HC1.  BiOCl  is  white  ;  insoluble  in  tartaric  acid, 
;  changed  by  I^S  into  Bi2S3. 

may  precipitate  (BiO)2SO4  under  the  conditions  noted 
in  the  preceding  paragraph  ;  soluble  in  acids. 

Na^nOg  in  alkaline  solution  (SnCl2  +  NaOH  in  excess)  pro- 
duces a  black  precipitate,2  the  Na2SnO2  being  oxidized  toNa2SnO3. 

1  The  precipitation  of  BiOCl  is  more  complete  than  that  of  the  other  basic  salts 
of  bismuth,  but  is  not  quantitative.     Classen  :  Ber.  d.  chem.  Ges.,  23,  940  (1890). 

2  The  composition  of  this  black  substance  is  uncertain.     Some  chemists  main- 
tain that  it  is  BiO,  while  others  believe  that  it  is  metallic  bismuth  in  a  colloidal 
form.    For  a  discussion  of  the  question,  see  Vanino  and  Treubert  :  Ber.  d.  chem. 
Ges.,  31,  1113,  2267  (1898);  32,  1072  (1899);  Schneider:  J.  prakt.  Chem.,  58, 
562  (1898);  60,  524  (1899);  Lottermoser  :  J.  prakt.  Chem.,  59,  489  (1899). 

Tanatar,  who  has  made  BiO  in  a  different  way,  states  that  the  black  substance 
formed  in  the  above  reaction  is  not  homogeneous.  Ztschr.  anorg.  Chem.,  27,  43^ 
(1901). 


ANALYSIS  OF  DIVISION  A 


27 


DIVISION  A 


METHOD  OF  ANALYSIS 

Boil  the  sulphides,  obtained  as  directed  on  page  40,  in  an  evaporator  with  a 
few  cubic  centimeters  of  HNO$  until  brown  fumes  cease  to  be  freely  evolved. 
Dilute  with  a  little  water  and  filter. 


Filtrate:   Lead,  Bismuth,  Copper,  Cadmium 

Residue  : 

Add  a  little  //2S04  and  evaporate  carefully  until  dense  white 

Mercury 

fumes  of  S03  appear.      Add  an  equal  volume  of  //2S04  and 

Boil   with   a 

filter. 

very  little  aqua 

Filtrate  :  Bismuth,  Copper,  Cadmium 
Add  NH4OH  until  strongly  alkaline.    A  deep 
blue   solution  proves  the  presence   of  copper. 
A  white  precipitate  indicates  bismuth.     Filter 

Residue:  Lead 

Wash,  warm 
the  precipitate 

regia.      Filter, 
boil  the  filtrate 
until    chlorine 
is  expelled,  and 
then  add  SnCla 

and  wash. 

with 

and  warm. 

NH4C2H302 

A   gray   or 

Filtrate:  Cop 
If  Copper  is 

per,  Cadmium 

If  Copper  is 

Precipitate  : 
Bismuth 

and  a  few  drops 
o  f      HC2H302 
and  filter.     To 

black    precipi- 
tate proves  the 
presence     of 

absent  : 

present  : 

Add  two  or 

the  filtrate  add 

mercury. 

Make  slightly 

Acidify  with 

three  drops  of 

K2Cr04. 

acid  with  HCI 

HCI    and    pre- 

HCI to  the  pre- 

A yellow  pre- 

and treat  with 

cipitate    with 

cipitate  in  the 

cipitate  soluble 

H2S. 

H2S.      Filter 

funnel    and 

in  NaOH  proves 

A  yellow  pre- 

and boil  the 

allow    the    fil- 

the presence  of 

cipitate  proves 

precipitate  im- 

trate   to    drop 

lead. 

* 

the  presence  of 

mediately  with 

into  a   beaker 

cadmium. 

//2S04.     Filter, 

of  water. 

rejecting  the 

A  white  pre- 

residue. Dilute 

cipitate  proves 

the   colorless 

the  presence  of 

filtrate  with  an 

bismuth. 

equal     volume 

of    water    and 

treat  with  H2S. 

A  yellow  pre- 

cipitate proves 

the  presence  of 

cadmium. 

28  HYDROGEN   SULPHIDE  GKOUP 

DISCUSSION 

Mercury  is  distinguished  from  the  other  members  of  Division  A 
of  this  group  by  the  insolubility  of  its  sulphide  in  HNOy  The 
black  sulphide  of  mercury,  HgS,  may  be  changed  to  the  white 
compound  Hg(NO3)2-2  HgS,  but  this  also  is  insoluble.  Lead 
that  is  present  in  the  solution  to  be  analyzed  will  not  be  pre- 
cipitated completely  in  the  hydrochloric  acid  group,  but  will 
appear  as  lead  sulphide  in  Division  A  of  the  hydrogen  sulphide 
group.  (If  the  precipitation  takes  place  from  a  strong  hydro- 
chloric acid  solution,  a  brick-red  compound,  PbCl2-2PbS,  may 
be  precipitated.  This  will  be  changed  to  the  black  sulphide 
either  by  further  treatment  with  hydrogen  sulphide  or  by  the 
action  of  ammonium  polysulphide  upon  the  group  precipitate.) 
In  the  treatment  of  the  sulphides  of  Division  A  of  this  group 
with  nitric  acid,  the  greater  part  of  the  lead  sulphide  is  usually 
changed  to  the  nitrate  which  dissolves.  (A  portion  may  be 
oxidized  to  the  sulphate.  The  latter  reaction  may  predominate 
if  the  boiling  is  prolonged  or  if  concentrated  acid  is  employed.) 
A  partial  precipitation  of  bismuth  as  a  basic  nitrate  may  occur 
when  the  nitric  acid  solution  is  diluted  before  filtration.  If 
this  is  the  case,  it  is  well  to  redissolve  the  precipitate  by  the 
addition  of  a  little  dilute  nitric  acid. 

Lead  is  distinguished  from  bismuth  and  cadmium  by  the 
insolubility  of  its  sulphate  in  //2$04.  As  lead  sulphate  is 
somewhat  soluble  in  nitric  acid,  this  acid  is  expelled  by  heat- 
ing the  solution  with  sulphuric  acid.  If  lead  is  not  entirely 
precipitated  at  this  point,  it  may  appear  as  the  sulphide  in 
the  final  test  for  cadmium.  Since  nitric  acid  is  more  easily 
decomposed  by  heat  than  is  sulphuric  acid,  it  may  be  assumed 
that  the  former  has  been  completely  removed  when  the  sulphuric 
acid  begins  to  decompose  into  water  and  sulphur  trioxide. 
Dilute  sulphuric  acid  and  not  water  is  used  in  diluting  this  solu- 
tion, because  water  might  precipitate  a  basic  sulphate  of  bismuth. 


ANALYSIS  OF  DIVISION  A  29 

The  separation  of  "bismuth  from  copper  and  cadmium  is  based 
on  the  insolubility  of  its  hydroxide  in  ammonium  hydroxide,  the 
hydroxides  of  copper  and  cadmium  being  soluble  in  that  reagent. 

If  a  deep  blue  color  is  imparted  to  the  solution  by  the  addi- 
tion of  ammonium  hydroxide  in  excess,  the  presence  of  copper 
is  conclusively  proved.  (In  case  it  is  desired  to  test  for  such 
very  small  quantities  of  copper l  as  might  by  this  method  escape 
detection,  a  portion  of  the  ammoniacal  solution  may  be  evapo- 
rated to  small  bulk,  acidified  with  acetic  acid,  and  the  solution 
tested  for  copper  with  potassium  ferrocyanide.) 

The  separation  of  copper  from  cadmium  is  based  on  the  solu- 
bility of  cadmium  sulphide  in  //2S04,  copper  sulphide  being 
insoluble  in  that  reagent.  The  sulphides  must  be  filtered  off 
and  treated  with  the  acid  immediately  or  some  of  the  copper 
sulphide  will  be  oxidized  by  the  air  to  sulphate  which  will 
pass  into  solution. 

Some  of  the  double  compounds  that  are  first  formed  when 
hydrogen  sulphide  acts  on  a  solution  of  a  mercuric  salt  are 
soluble  in  nitric  acid.  If,  therefore,  the  precipitation  is  inter- 
rupted before  these  compounds  are  completely  transformed  into 
mercuric  sulphide,  some  mercury  will  pass  into  the  filtrate  upon 
treatment  with  nitric  acid.  In  the  subsequent  treatment  of 
this  filtrate  the  mercury  in  such  case  will  still  be  in  solution 
after  an  excess  of  ammonium  hydroxide  is  added,  and  will  be 
precipitated  by  hydrogen  sulphide  with  the  sulphides  of  copper 
and  cadmium.  A  portion  of  the  mercuric  sulphide  thus  formed 
may  be  dissolved  when  the  sulphides  of  copper  and  cadmium 
are  treated  with  sulphuric  acid,  and  will  then  be  precipitated 
as  black  mercuric  sulphide  in  the  final  test  for  cadmium.  This 
difficulty  is  not  encountered  in  case  the  group  precipitate  has 
been  treated  with  ammonium  polysulphide,  because  this  reagent 
transforms  these  intermediate  compounds  into  mercuric  sulphide. 
It  should  be  remembered  that  a  black  precipitate  in  the  final 
test  for  cadmium  may  be  lead  sulphide  (see  page  28). 

1  See  also  page  43. 


30  HYDROGEN  SULPHIDE   GROUP 

DIVISION  B 

PRELIMINARY  REACTIONS 

ARSENIC:  Reactions  of  solutions  of  arsenious  compounds. 

KOH  or  NaOH  produces  no  precipitate. 

NH4OH  produces  no  precipitate, 
produces  no  precipitate. 

precipitates  from  arsenious  solutions  containing  hydrochloric  acid 
As2S3 ,  lemon  yellow ;  almost  insoluble  in  warm  concentrated  HC1 ;  oxi- 
dized by  a  crystal  of  KC103  and  hot  HCI,  or  by  hot  HNO^  to  H3As04 
which  is  soluble,  sulphur  being  set  free  at  the  same  time ;  As2S3  is  sol- 
uble in  (NH4)2SX  forming  sulpho-salts,  such  as  ammonium  sulpho-arsenate, 
(NH4)3AsS4,  from  which  dilute  HCI  precipitates  As2S5. 

(A  portion  of  the  As2S3  precipitate  should  be  dried  for  use 
in  the  KCN  fusion  test  described  below.  To  do  this  lay  the 
filter  containing  the  remainder  of  the  precipitate  upon  a  piece 
of  wire  gauze.  Place  this  wire  gauze  high  above  a  low  Bunsen 
flame  and  dry  the  filter  and  precipitate  thoroughly,  but  do  not 
allow  the  paper  to  char.) 

(NH4)2S  precipitates  As2S3  from  solutions  of  H3AsO3  acidified 
with  hydrochloric  acid.  The  reagent  here  is  H2S,  formed  by 
the  action  of  (NH4)2S  on  HCI.  As2S3  is  soluble  in  (NH4)2S, 
forming  (NH4)3AsS3. 

HCI  produces  no  precipitate. 

HjSC^  produces  no  precipitate. 

Nascent  hydrogen.  If  a  slightly  acid  solution  of  any  compound  of 
arsenic  is  introduced  into  a  hydrogen  generator  (see  Gutzeit  test),  arsine, 
AsH3,  a  colorless,  poisonous  gas,  is  formed. 

When  arsine 1  is  led  into  a  solution  of  AgNO3  a  finely  divided 
black  precipitate  of  metallic  silver  is  produced,  and  the  arsine  is 

1  The  hydrogen  generator  used  in  this  experiment  consists  of  a  100  cc.  flat- 
bottom  flask  provided  with  a  two-hole  rubber  stopper.  Through  one  opening 
of  the  stopper  passes  a  delivery  tube  ending  just  below  it  and  so  bent  above  the 
stopper  that  it  projects  downward  at  an  angle  of  45°  and  extends  about  4  cm. 


AKSENIC  31 

at  the  same  time  changed  to  H3AsO3.  The  oxygen  necessary 
for  this  oxidation  seems  to  be  furnished  by  the  water.1  The 
silver  may  be  filtered  off  and  the  arsenic  in  the  filtrate  precipi- 
tated as  As2S3  by  H2S.  But  this  filtrate  usually  contains  some 
AgNO3  that  has  not  been  acted  upon  by  the  arsine,  and  conse- 
quently Ag2S  would  be  precipitated  with  As2S3  when  the  filtrate 
is  treated  with  H2S.  It  is  therefore  necessary  to  remove  the 
silver  in  the  filtrate,  by  precipitating  it  with  HC1  and  filtering 
off  the  AgCl,  before  H2S  is  added. 

Gutzeit  test  (modified).  When  arsine  acts  upon  a  crystal  of  AgN03 
there  is  first  formed  a  yellow  compound  which  is  said  to  be  AsAg3'3AgN03. 
If  the  amount  of  arsine  is  large,  the  yellow  compound  appears  only  for  an 
instant,  and  turns  black  almost  immediately,  metallic  silver  and  arsenious 
acid  being  formed.  The  oxygen  necessary  for  the  production  of  arsenious 
acid  seems  to  be  furnished  by  the  moisture  in  the  gas. 

In  this  experiment  it  is  convenient  to  use  a  test  tube  as  a  hydrogen  gen- 
erator. To  ascertain  the  purity  of  the  reagents,  a  blank  test  should  first 
be  made  as  follows  :  —  Place  a  fragment  of  zinc  in  a  dry  test  tube  and  add 
through  a  funnel  a  few  cubic  centimeters  of  dilute  I^SC^.  Introduce  into 
the  test  tube  a  loosely  fitting  plug  of  absorbent  cotton  about  3  cm.  long  and 
push  this  down  into  the  tube  until  its  upper  surface  is  about  2  cm.  below 
the  mouth  of  the  tube.  Above  this  place  a  second  plug  of  cotton,  the 
lower  end  of  which  has  been  moistened  with  a  solution  of  lead  ammonium 
acetate  to  absorb  any  H2S  that  may  be  evolved  from  the  reagents.  Cover 

beyond  the  neck  of  the  flask.  Through  the  other  opening  passes  a  funnel  tube 
reaching  nearly  to  the  bottom  of  the  flask.  In  using  the  generator,  place  in 
the  flask  some  zinc  and  a  small  piece  of  platinum  foil,  insert  the  stopper,  and 
add  through  the  funnel  tube  from  5  to  15  cc.  of  dilute  sulphuric  acid,  the 
volume  of  acid  being  dependent  upon  the  amount  of  arsenic  to  be  introduced. 
Join  to  the  end  of  the  delivery  tube  by  means  of  a  short  piece  of  rubber  tubing 
a  straight  glass  tube,  drawn  out  at  the  lower  end  and  of  sufficient  length  to 
reach  to  the  bottom  of  a  test  tube.  Fill  a  test  tube  half  full  with  a  solution  of 
AgNOa  and  slip  it  over  this  delivery  tube.  Now  introduce  through  the  funnel 
tube  the  solution  which  is  to  be  tested,  adding  but  a  little  at  a  time.  Never 
allow  the  gases  which  are  evolved  to  pass  directly  into  the  atmosphere,  and 
keep  all  flames  away  from  the  apparatus. 

1  The  products  of  this  reaction  are  usually  said  to  be  arsenious  and  nitric 
acids  a"nd  metallic  silver.  It  has  been  shown  in  Lunge's  laboratory  that  some 
nitrous  acid  also  is  formed.  Marchlewski :  Eer,  d.  chem.  Ges.,  24,  2269  (1891). 


32  HYDKOGEN  SULPHIDE  GEOUP 

the  mouth  of  the  tube  with  a  piece  of  filter  paper  and  fold  this  down 
snugly  around  the  edge.  On  this  paper  place  a  small  crystal  of  AgN03 
and  a  fragment  of  Pb(C2H302)2.  The  dry  cotton  is  used  to  absorb  mois- 
ture from  the  gas.  In  case  H2S  is  formed,  the  crystal  of  Pb(C2H302)2 
on  the  paper  will  show  whether  it  has  been  completely  absorbed  by  the 
moistened  plug.  If  the  reagents  are  pure  and  the  experiment  is  properly 
performed,  neither  of  the  crystals  on  the  paper  should  change  color. 

Having  thus  tested  the  apparatus,  remove  the  plugs  and  introduce  a 
few  drops  of  H3As03.  Quickly  arrange  the  apparatus  as  before  and 
observe  the  color  changes  in,  the  silver  nitrate  crystal. 

Gatehouse  test  (modified).  This  differs  from  the  Gutzeit  test  only  in 
that  the  source  of  hydrogen  is  aluminum  and  KOH.  The  reaction  may 
be  hastened  by  gently  warming  the  tube  until  bubbles  begin  to  appear  on 
the  surface  of  the  aluminum.  The  lead  salts  are  not  needed,  but  a  plug 
of  absorbent  cotton  should  be  inserted  to  dry  the  gas. 

KCN.  Dry  As2O3  or  As2S3,  when  fused  in  a  bulb  tube  with 
a  dry  mixture  of  three  parts  Na2CO3  and  one  part  KCN,  is 
reduced  to  metallic  arsenic  which  is  deposited  in  the  cooler 
part  of  the  tube  as  a  shining  black  mirror.  If  the  amount  of 
arsenic  present  is  small,  the  mirror  has  a  brownish  color.  KCN 
acts  as  a  reducing  agent  here  because  it  unites  with  oxygen  to 
form  KCNO,  or  with  sulphur  to  form  EONS. 

Reinsch  test.  If  a  bright  strip  of  copper  foil  is  introduced  into  an 
arsenious  solution  to  which  has  been  added  from  one-fourth  to  one-sixth 
of  its  volume  of  concentrated  HC1,  and  the  mixture  is  boiled,  arsenic  is 
deposited  on  the  copper  as  a  gray  coating.  If  the  foil  is  then  removed, 
washed  with  water,  dried  by  pressing  between  pieces  of  filter  paper, 
introduced  into  a  bulb  tube  and  heated,  a  sublimate  of  crystalline  As203 
is  formed.  This  should  further  be  identified  by  an  additional  test  for 
arsenic.  If  the  Gatehouse  test  is  to  be  employed,  the  portion  of  the  tube 
that  contains  the  sublimate  is  cut  off  and  a  strip  of  aluminum  foil  is 
passed  through  it  and  bent  over  at  the  ends  to  hold  the  foil  in  place. 
The  experiment  is  then  performed  in  the  manner  already  described  (see 
above). 

Bettendorff  test.  When  a  mixture  of  equal  parts  of  a  concentrated 
solution  of  SnCLj  and  concentrated  HC1  is  boiled  with  a  few  drops  of 
H3As03,  the  latter  is  reduced  to  metallic  arsenic. 

Bettendorff  test  (modified).  If  a  strip  of  tin  foil  is  placed  in  a  mod- 
erately dilute  solution  of  stannous  chloride  containing  hydrochloric  acid, 


y 

ARSENIC  33 

and  a  few  drops  of  a  solution  of  arsenious  acid  are  then  added  and  the 
liquid  heated  to  boiling,  the  arsenious  acid  is  reduced  to  metallic  arsenic 
which  in  part  is  deposited  unevenly  on  the  foil  and  in  part  remains  sus- 
pended in  the  solution,  imparting  to  it  a  brown  color. 

Reactions  of  a  solution  of  arsenic  acid. 

KOH  or  NaOH  produces  no  precipitate. 

NH4OH  produces  no  precipitate. 

Na2C03  produces  no  precipitate. 

I^S.  If  a  current  of  hydrogen  sulphide  is  rapidly  passed  through  a 
solution  of  arsenic  acid,  As2S5  is  slowly  precipitated.1  The  precipitation 
is  complete  and  much  more  rapid  when  a  relatively  large  quantity  of  hydro- 
chloric acid  is  present.2  If  hydrogen  sulphide  is  slowly  passed  through  a 
hot  solution  of  arsenic  acid  that  contains  but  a  moderate  amount  of  hydro- 
chloric acid,  a  different  reaction  takes  place :  arsenic  acid  is  reduced  to 
arsenious  acid  and  arsenic  trisulphide  is  precipitated,  mixed  with  sulphur. 
Arsenic  pentasulphide  resembles  the  trisulphide  in  color  and  in  its  behavior 
toward  most  reagents. 

(NH4)2S  precipitates  As2S5  from  a  solution  of  H3 AsO4  acidified 
with  HC1.  The  reagent  here  is  the  H2S  formed  by  the  action 
of  (NH4)2S  on  HC1.  As2S5  is  soluble  in  (NH4)2S><  forming 
sulpho-salts  such  as  (NH4)3AsS4. 

HC1  produces  no  precipitate. 

H2S04  produces  no  precipitate. 

(NH4)2Mo04,  ammonium  molybdate,  added  in  excess  to  a  solution 
of  arsenic  acid  containing  nitric  acid  precipitates  ammonium  arseno- 
molybdate,  yellow ;  less  soluble  in  HN03  than  in  water.  Precipitation  is 
best  obtained  by  adding  a  few  drops  of  HN03  and  a  few  drops  of  H3As04 
to  a  small  test  tube  half  full  of  (NH4)2Mo04.  The  mixture  should  then 
be  warmed  but  should  not  be  heated  above  70°,  lest  white  Mo03  be 
precipitated. 

1  Brauner  and  Tomicek  :  Monatshefte,  8,  624  (1887). 

2  From  a  cold  solution  containing  two  or  more  parts  of  concentrated  hydro- 
chloric acid  (sp.  gr.  1.20)  to  one  part  of  water  a  moderate  amount  of  arsenic 
acid  is  completely  precipitated  as  As2Ss  in  from  half  an  hour  to  an  hour.     This 
has  been  made  the  basis  of  a  method  for  the  quantitative  determination  of 
arsenic.     Under  the  above  conditions  lead,  bismuth,  cadmium,  and  antimony 
are  not  precipitated.     Neher  :  Ztschr.  anal  Chem.,  32,  45  (1893). 


34  HYDROGEN  SULPHIDE   GKOUP 

MgCl2.  When  this  reagent  is  added  to  a  solution  of  arsenic 
acid  or  an  arsenate,  the  solution  having  first  been  made  alkaline 
with  NH4OH,  there  is  formed  MgNH4AsO4,  a  white  crystalline 
precipitate.  If  the  amount  of  arsenic  present  is  small,  the  pre- 
cipitate forms  only  after  some  time.  (Compare  the  precipita- 
tion of  MgNH4PO4,  page  T4.) 

FeS04,  Na^Og,  or  (NH4)2S03  when  boiled  with  a  solution  of  H3As04 
that  has  been  acidified  with  H2S04  or  HC1  reduces  it  to  H3As03.  The 
sulphites  are  best  used  in  the  solid  form. 

Nascent  hydrogen  (zinc  and  sulphuric  acid)  reduces  H3AsO4 
first  to  H3AsO3,  and  this  is  then  further  reduced  to  AsH3; 
complete  reduction,  therefore,  takes  much  more  time  than  when 
an  arsenious  solution  is  introduced  into  the  generator. 

Gutzeit  test  (modified).     See  preceding  paragraph. 

Gatehouse  test  (modified).  Nascent  hydrogen  evolved  by  the  action 
of  aluminum  on  potassium  hydroxide  does  not  reduce  arsenic  acid. 

In  the  Bettendorff  test,  Bettendorff  test  (modified),  and  Reinsch  test 
reduction  takes  place  less  rapidly  than  when  the  arsenic  is  in  the  triva- 
lent  condition.  * 

ANTIMONY  :  Reactions  of  solutions  of  antimonious  salts. 

KOH  or  NaOH  precipitates  Sb(OH)3 ;  soluble  in  excess  of  KOH, 
forming  Sb(OK)3. 

NH4OH  produces  a  similar  precipitate. 

Na2C03  produces  a  white  precipitate,  probably  Sb(OH)3;  soluble 
in  large  excess  of  the  reagent. 

H9S  precipitates  from  moderately  acid  solutions  of  antimonious  salts 
Sb2S3,  orange  red ;  soluble  in  warm,  concentrated  HC1 ;  oxidized  by  hot 
HNOfr  forming  sulphur  and  H3Sb04  which  is  soluble.  Sb2S3  is  soluble  in 
(NH4)2SX  forming  sulpho-antimonates,  as  (NH4)3SbS4,  from  which  dilute 
acids  precipitate  Sb2S5 ,  orange  red.  The  latter  is  also  precipitated  when 
acid  solutions  of  antimonic  salts  are  treated  with  H2S.  Sb2S5  resembles 
Sb9S3  in  its  behavior  toward  most  reagents ;  when  treated  with  hot 
concentrated  hydrochloric  acid,  however,  it  is  changed  to  SbCl3  with  the 
separation  of  sulphur.  f 

(NH4)2S  precipitates  Sb2S3  from  antimonious  solutions  acidified 


ANTIMONY  35 

with  hydrochloric  acid.  The  reagent  here  is  the  H2S  formed 
by  the  action  of  (NH4)2S  on  HC1.  Sb2S3  is  soluble  in  (NH4)2S, 
forming  (NH4)3SbS3. 

HC1  precipitates,  from  solutions  that  do  not  contain  too  much  free  acid, 
SbOCl,  white;  soluble  in  dilute  inorganic  acids  or  in  tartaric  acid.  The 
precipitate  is  best  obtained  by  adding  a  small  amount  of  the  antimonious 
solution  to  a  large  quantity  of  water  that  has  been  acidified  with  a 
little  HC1. 

H20.  If  a  few  drops  of  a  solution  of  SbCl3  that  does  not  contain  too 
much  free  HC1  are  added  to  a  relatively  large  quantity  of  water,  SbOCl 
is  precipitated.  This  is  changed  directly  to  Sb2S3  when  treated  with 


H2S04  produces  no  precipitate  from  acid  or  neutral  solutions 
of  antimonious  salts. 

Nascent  hydrogen.  If  a  slightly  acid  solution  of  a  compound  of  anti- 
mony is  placed  in  a  platinum  spoon  together  with  a  small  piece  of  zinc,  a 
portion  of  the  antimony  will  be  deposited  upon  the  platinum  in  metallic 
form  and  will  appear  as  a  black  stain.  This  deposit  is  only  slightly 
soluble  in  cold  //£/,  but  is  easily  dissolved  by  a  few  drops  of  fuming 
HN03,  the  latter  reagent  oxidizing  it  to  H3Sb04.  The  excess  of  HN03 
may  completely  be  removed  by  adding  to  the  solution  a  few  crystals  of 
tartaric  acid  and  then  warming  the  mixture.  The  antimony  is  precipitated 
in  the  form  of  a  sulphide  when  the  residual  solution  is  diluted  with  a 
little  water  and  treated  with  H2S. 

If  an  acid  solution  of  a  compound  of  antimony  is  introduced 
into  a  hydrogen  generator  (see  page  30)  there  may  be  produced 
by  the  action  of  the  nascent  hydrogen  both  antimony  and  stibine. 
The  antimony  will  be  deposited  upon  the  platinum  foil  in  the 
generator.  The  properties  of  this  metallic  antimony  have  been 
described  in  the  preceding  paragraph.  Stibine,  SbH3,  is  a  color- 
less, poisonous  gas.  If  this  gas  is  led  into  a  solution  of  AgNO3 
there  is  formed  SbAg3,  black.  If  this  substance  is  washed  with 
water  and  boiled  with  tartaric  acid,  it  is  decomposed  with  the 
formation  of  silver  and  a  soluble  compound  of  antimony.  The 
antimony  is  precipitated  in  the  form  of  a  sulphide  when  H2S  is 
passed  through  the  nitrate. 

•     i 

- 


36  HYDKOGEN  SULPHIDE   GROUP 

[The  following  tests  are  made  as  described  on  pages  31  and  32,  except 
that  SbClo  is  used  instead  .of  HoAsOo.l 

o  o  A  J 

Gutzeit  test  (modified).  When  SbH3  acts  upon  a  crystal  of  AgN03  it 
is  possible  that  there  may  first  be  formed  a  yellow  compound  analogous  to 
that  produced  by  the  action  of  AsH3 .  In  practice,  however,  this  is  seldom 
seen,  black  SbAg3  usually  forming  at  once. 

Gatehouse  test  (modified).  Metallic  antimony  is  deposited  in  the  test- 
tube  generator  chiefly  on  the  aluminum,  and  no  SbH3  is  formed. 

Bettendorff  test.    Antimony  is  not  precipitated. 

Bettendorff  test  (modified).  Antimony  is  deposited  as  a  velvety  black 
coating  which  is  evenly  distributed  over  the  entire  surface  of  the  tin  foil. 

TiN:  Reactions  of  solutions  of  stannous  salts. 

KOH  or  NaOH  precipitates  Sn(OH)2,  white ;  soluble  in  excess, 
forming  stannites,  as  Sn(OK)2. 

NH4OH  precipitates  Sn(OH)2 ;  insoluble  in  excess. 

Na2C03  precipitates  Sn(OH)2 ;  soluble  ^n  large  excess  of  the 
reagent. 

H2S  precipitates  SnS,  dark  brown  ;  soluble  iBwarm  concentrated  HC1 ; 
soluble  in  (NH4)2SX,  forming  sulpho-stannates,  a^(NH4)2SnS3,  from  which 
a  dilute  acid  precipitates  SnS2,  yell^F 

(NH4)2S  precipitates  SnS  from  stannous  sdffetions  acidified  with 
HC1.  The  reagent  here  is  H2S,  formed  by  the  action  of  (NH4)2S 
on  HC1.  SnS  is  insoluble  or  nearly  so  in  colorless  (NH4)2S. 

HC1  produces  no  precipitate. 

H2S04  produces  no  precipitate. 

HgCl2  is  reduced  by  SnCl2  in  hydrochloric  acid  solution  to  HgCl,  white  ; 
if  an  excess  of  SnCl2  is  added,  some  of  the  HgCl  is  reduced  to  black 
metallic  mercury,  and  the  resultant  mixture  usually  has  a  gray  color. 

Nascent  hydrogen.  If  a  solution  of  a  compound  of  tin  containing  a 
small  amount  of  HC1  is  acted  upon  by  nascent  hydrogen  (zinc  and  sul- 
phuric acid),  it  will  be  reduced  to  metallic  tin,  which  will  appear  as  a 
spongy  mass  adhering  chiefly  to  the  zinc.  Metallic  tin  dissolves  when 
boiled  with  HCI,  forming  SnCl2.  The  action  is  hastened  by  placing  a 
small  piece  of  platinum 'in  contact  with  the  tin. 

KC103.  When  a  crystal  of  this  substance  is  added  to  a  solution  of 
SnCl2  contajryngry/tf',  and  the  mixture  is  warmed,  SnCl4  is  formed. 


TIN  37 

HN03  oxidizes  metallic  tin  to  SnO2  or  H2SnO3,  white,  amor- 
phous substances  difficultly  soluble  in  acids.1 

Reactions  of  acid  solutions  of  stannic  salts. 

(The  following  reactions  are  those  of  a  solution  of  a  stannic 
salt  that  has  been  prepared  directly  from  a  stannous  salt  by 
treating  it  with  KC1O3  and  HC1.  A  solution  of  a  stannic  salt 
made  from  metastannic  acid  behaves  somep'hat  differently. 
Since  hydrogen  sulphide  reacts  in^the  same  way  with  either 
solution,  it  is  considered  sufficient  for  Jhg  purposes  of  this 
manual  to  introduce  in  th&r  preliminary  reactions  only  those 
of  such  stannic  soluti^^p  as  are  usually  encountered  in  the 
course  of  analysis.) 

KOH  or  NaOH  precipitates  stannic  acid,  Sn(OH)4,  white,  gelat- 
inous;  solub^r  iir  KOH  or  NaOH,  forming  stannates,  as 
Sn(OK)4.  Upon  neyttnt-liziiig  j^.j^olution  of  a  stannate  with  a 
dilute  ack^n(0TTA-iJ-1-1 ^  ^—  - 

NH4OH  precipitat  ^^ 

Na2003  precipitat^^^Bnuc  acid-;  soluble  in  excess. 

H2S  precipitates  ylow  SnS2  from  solutions  Containing  a  moderate 
amount  of  HC1 ;  soltrole  in  warm  concentrated  'HC1 ;  soluble  in  (NH4)2SX 
to  form  sulpho-stannates,  as  (NH4)2SnS3/  * 

(NH4)2S  precipitates  SnS2  from  stannic  solutions  acidified  with 
HC1.  The  r^|ent  here  is  H2S,  formed  by  the  action  of  (NH4)2S 
on  HC1.  Sn¥2  is  soluble  in  (NH4)2S,  forming  (NH4)2SnS3.x 

HC1  produces  no  precipitate  from  soluticra  of  moderate  con- 
centration. 

H2S04  produces  no  precipitate  from  solutions  tof  ordinary 
concentration.  From  very  dilute  solutions  a  basic  sulphate  is 
precipitated. 

HgC^  produces  no  precipitate. 

Nascent  hydrogen  reduces  stannic  solutions  to  metallic  tin. 

1  a-stannic  acid  is  easily  soluble  in  inorganic  acids.  This^acid  is  not  formed 
in  the  treatment  above  described. 


38  HYDROGEN  SULPHIDE  GROUP 

PRECIPITATION  AND  SEPARATION  OF  DIVISIONS  A  AXD  B 
PREPARATION  OF  THE  SOLUTION 

The  solution  from  which  this  group  is  to  be  precipitated  should 
be  free  from  nitric  acid  and  should  contain  about  one  volume 
of  HCI  to  ten  volumes  of  the  water  solution. 

If  nitric  acid  has  been  used  to  dissolve  the  original  substance, 
evaporate  to  srafcll  bulk  and  boil  with  HCI  until  the  nitric 
acid  has  been  reduced  and  tlie  odor  of  chlorine  is  no  longer 
noticeable. 

If  hydrochloric  acid  has  been  used  to  dissolve  the  original 
substance  or  lo  remove  nitric  acid/ltf d  water  to  the  solution 
until  the  desired  concentration  is  reacheu  otherwise  add  to  the 
filtrate  from  the  hydrochloric  acid  group,  \  c  to  the  solution 
itself  if  it  has  been  found  to  contain  no  members  of  that  group, 
about  one-tenth  its  volume  of //£/. 

\ 

PRECIOTATION 

| 

Heat  the  solution  to  boiling  and  pass  through  the  hot  solu- 
tion a  slow1  current  of  hydrogen  sulphide! until  precipitation 
is  complete,  the  solution  is  cool,  and  no  further  change  of 
color  is  observed  in  the  precipitate.2  To  determine  the  point 

1  The  gas  should  pass  through  the  solution  at  such  a  rate  that  the  bubbles 
may  easily  be  counted. 

2  It  has  been  suggested  that  complete  precipitation  may  be  secured  in  less 
time  and  with  less  wq(0B  of  H2S  by  using  the  following  arrangement.     The 
vessel  in  which  precipitation  takes  place  is  an  Erlenmeyer  flask  of  such  a  size 
that  it  is  not  more  than  one-third  filled  by  the  solution.     The  flask  is  furnished 
with  a  rubber  stopper,  through  which  passes  a  glass  tube  reaching  to  the  bottom 
of  the  flask  and  bent  at  a  right  angle  above  the  stopper.     Introduce  into  the 
flask  the  solution  prepared  as  above  directed  and  heat  it  to  boiling.     Insert  the 
stopper  loosely,  having  withdrawn  the  tube  so  that  its  lower  end  is  just  above 
the  surface  of  the  liquid.     Attach  the  other  end  of  the  tube  by  six  inches  of 
rubber  tubing  to  the  HaS  generator  and  pass  a  rapid  current  of  gas  through 
the  flask  for  a  mogaent  to  expel  the  air.     Without  shutting  off  the  gas  insert  the 
stopper  firmly  and  push  down  the  tube  until  it  reaches  nearly  to  the  bottom  of 


PRECIPITATION  39 

of  complete  precipitation,  filter  a  small  portion  into  a  test 
tube,  dilute  with  one-half  its  volume  of  water,  and  treat  with 
hydrogen  sulphide  for  a  few  minutes. 

In  a  solution  known  to  contain  no  members  of  subsequent 
groups,  filter  and  reject  the  nitrate.  Otherwise  reserve  the 
filtrate  for  the  analysis  of  the  following  groups. 

If  no  precipitate  is  produced  by  hydrogen  sulphide,  pass  to 
the  ammonium  sulphide  group.  (If  oxidizing  agents  have  not 
been  removed,  free  sulphur  may  be  formed.  See  page  40.) 

Wash  the  precipitate  thoroughly  with  hot  water  until  a  few 
drops  of  the  wash  water  impart  but  a  slight  turbidity  to  a  little 
silver  nitrate  solution. 

If  Division  A  is  known  to  be  absent,  treat  the  precipitate  as 
directed  on  page  44. 

If  Division  B  is  known  to  be  absent,1  or  if  both  divisions  may 
be  present,  proceed  as  follows  : 

SEPARATION  OF  DIVISIONS  A  AND  B 

Transfer  a  small  portion  of  the  precipitate  to  an  evaporating 
dish,  add  a  few  cubic  centimeters  of  ammonium  polysulphide 
and  warm  for  a  moment.  If  this  portion  of  the  precipitate 
dissolves  completely,  Division  A  is  absent  and  the  remainder 
of  the  precipitate  should  be  treated  as  directed  on  page  44. 

If  the  portidn  of  the  precipitate  that  is  treated  with  ammo- 
nium polysulphide  is  not  completely  dissolved,  the  presence  of 
Division  A  is  indicated.  In  such  case  treat  the  remainder  with 

the  flask.  Shake  the  flask  occasionally.  After  a  short  time  disconnect  the  flask 
from  the  generator  and  cool  its  contents  by  holding  it  under  a  stream  of  cold 
water.  Now  dilute  the  liquid  in  the  flask  with  an  equal  volume  of  cold  water 
and  repeat  the  treatment  with  H2S  in  the  manner  described.  See  Graebe :  Ber.  d. 
chem.  Ges.,  31,  2081  (1898).  Precipitation  will  usually  be  found  to  be  complete 
after  this  second  treatment  with  the  gas  has  pontinued  for  a  few  minutes. 

1  Even  when  it  is  known  that  members  of  Division  B  are  absent,  it  is  well  to 
treat  the  precipitate  with  ammonium  polysulphide  in  order  to  change  any  inter- 
mediate sulpho-chlorides  of  lead  or  mercury  into  the  sulphides. 


40  HYDROGEN  SULPHIDE  GROUP 

ammonium  polysulphide  (10  cc.  will  usually  suffice)  and  warm 
for  about  three  minutes  with  occasional  stirring.  The  mixture 
should  not  be  allowed  to  boil.  Filter  while  warm,  and  analyze 
the  residue  as  directed  for  Division  A  on  page  27. 

The  filtrate  may  contain  members  of  Division  B.  Add  HC1 
until  the  solution  is  milk-white  from  the  separation  of  finely 
divided  sulphur,  or  until  further  acidification  produces  no  more 
precipitation.  If  no  flocculent  or  colored  precipitate  is  formed, 
members  of  Division  B  are  absent.  If  a  precipitate  appears,  filter, 
reject  the  filtrate,  and  treat  the  precipitate  as  directed  on  page  44. 

DISCUSSION 

The  members  of  the  hydrogen  sulphide  group  are  distin- 
guished from  those  of  the  hydrochloric  acifl  group  by  the 
greater  solubility  of  their  chlorides  in  dilute  hydrochloric  acid, 
and  from  those  of  the  other  groups  by  the  relative  insolubility 
of  their  sulphides  in  the  same  reagent. 

If  nitric  acid  is  present  in  any  considerable  amount  in  the 
solution  that  is  treated  with  hydrogen  sulphide,  the  members  of 
the  hydrogen  sulphide  group  will  not  be  completely  precipitated 
until  most  of  that  acid  has  been  reduced  by  the  hydrogen  sul- 
phide. Other  oxidizing  agents,  such  as  permanganates,  chro- 
mates,  ferric  salts,  etc.,  will  also  be  reduced.  The  free  sulphur 
that  is  formed  by  this  reduction  is  often  thrown  out  in  a  more 
or  less  flocculent  form  and  may  thus  lead  to  a  false  conclusion 
as  to  the  presence  of  members  of  this  group.  It  may  also  make 
difficult  the  subsequent  treatment  of  the  precipitate  (filtration, 
etc.).  For  this  reason  it  is  well  to  reduce  not  only  nitric  acid, 
but  also,  so  far  as  may  be,  any  other  oxidizing  agents  that  are 
present,  before  the  solution  is  treated  with  hydrogen  sulphide. 

The  treatment  with  hydrochloric  acid  prescribed  for  the 
removal  of  nitric  acid  will  also  decompose  nitrites,  chlorates, 
and  sulphites.  If  a  permanganate  or  chromate  is  present,  the 
reduction  may  be  hastened  by  the  addition  of  a  little  alcohol 


PRECIPITATION  41 

before  boiling  with  hydrochloric  acid,  but  in  such  case  the  boil- 
ing should  be  continued  until  the  excess  of  alcohol  is  removed. 
This  reduction  of  permanganates  and  chromates  is  further 
desirable,  since  these  salts,  if  present  in  relatively  large  amounts, 
will  form  black  or  colored  precipitates  when  reduced  by  hydro- 
gen sulphide.  Prolonged  boiling  of  arsenious  compounds  with 
hydrochloric  acid  should  be  avoided,  as  arsenious  chloride  is 
volatile. 

The  quantity  of  hydrochloricfacid  in  the  solution  that  is  to 
be  treated  with  hydrogen  sulphicrtPlfe  important.  Enough  of 
the  acid  must  be  present  to  prevent  the  precipitation  of  the 
members  of  the  ammonium  sulphide  group,  but  its  amount 
must  not  be  so  great  as  to  prevent  the  precipitation  of  those.' 
sulphides  of  the  hydrogen  sulphide  group  which  are  somewhat 
soluble  in  strong  hydrochloric  acid. 

Moreover,  the  presence  of  hydrochloric  acid  is  necessary  in 
order  to  insure  the  complete  precipitation  of  members  of 
Division  B.  From  alkaline  solutions  these  elements  are  pre- 
cipitated by  hydrogen  sulphide  incompletely  or  not  at  all, 
owing  to  the  formation  of  soluble  sulpho-salts.  When  hydrogen 
sulphide  is  passed  into  neutral  solutions  or  into  solutions  that 
contain  these  elements  in  the  form  of  acids,  if  no  other  acids 
or  salts  are  present,  pseudo-solutions1  of  the  sulphides2  are 
formed.  Hydrochloric  acid  prevents  the  formation  of  these 
pseudo-solutions  and  causes  the  sulphides  to  be  precipitated  in  a 
compact  form,  which  makes  filtration  easy.  The  concentration, 
one  part  of  HCI  to  ten  parts  of  solution,  has  been  found  to 
answer  the  conditions  most  satisfactorily. 

Precipitation  of  arsenic  pentasulphide  from  solutions  of 
arsenic  acid  takes  place  slowly  and  is  complete  only  after 
several  hours.  By  heating  the  solution  at  the  beginning  of  the 

1  Pseudo-solutions  are  discussed  in  the  Introduction,  page  12. 
2Schulze:  J.  prakt.   Chem.,  25,  431  (1882);  27,  320  (1883);  Brauner  and 
Tomicek :  Monatshefte,  8,  614  (1887). 


42  HYDROGEN  SULPHIDE  GEOUP 

treatment  with  hydrogen  sulphide,  conditions  are  made  favorable 
for  the  reduction  of  arsenic  acid  to  arsenious  acid,  and  from  the 
latter  arsenic  trisulphide  is  easily  precipitated.  (If  arsenic  acid 
is  known  to  be  present,  it  may  be  reduced  with  sulphur  dioxide, 
the  excess  of  the  latter  expelled  by  boiling,  and  the  solution 
then  treated  with  hydrogen  sulphide.  It  must  be  borne  in 
mind,  however,  that  in  this  reaction  sulphuric  acid  is  formed 
and  that  this  will  precipitate  lead  and  members  of  the  ammonium 
carbonate  group.) 

The  solution  should  be  cool  at  the  end  of  the  operation  in 
order  to  secure  the  complete  precipitation  of  the  sulphides 
soluble  in  hot  hydrochloric  acid. 

/  The  members  of  Division  B  of  the  hydrogen  sulphide  group 
are  distinguished  from  those  of  Division  A  by  the  solubility  of 
their  sulphides  in  ammonium  polysulphide,  sulpho-salts  being 
formed.  In  these  compounds  the  arsenic  and  antimony  are 
pentavalent  and  the  tin  tetravalent,  even  if  the  three  elements 
were  precipitated  as  their  lower  sulphides  by  hydrogen  sulphide. 
In  the  latter  case  it  may  be  assumed  that  the  excess  of  sulphur 
in  the  polysulphide  unites,  for  example,  with  SnS  to  form  SnS2, 
which  then  reacts  with  (NH4)2S  to  form  the  sulpho-salt. 

SnS  4-  S  =  SnS2  ; 

SnS2  +  (NH4)2S  =  (NH4)2SnS8. 

When  these  sulpho-salts  are  treated  with  an  acid  it  is  pos- 
sible that  the  corresponding  free  sulpho-acids  are  at  first 
formed,  as 

(NH4)2SnS3  +  2  HC1  =  2  NH4C1  -f  H2SnS3 ; 

but  if  they  are,  they  immediately  break  down  into  free  hydro- 
gen sulphide  and  the  sulphide  of  the  metal,  as 

H2SnS3  =  H2S  +  SnS2. 

The  separation  of  Divisions  A  and  B  by  ammonium  polysul- 
phide is  not  always  sharp.  Copper  sulphide  is  somewhat  soluble 


SEPARATION  OF  DIVISIONS  A  AND  B  43 

in  this  reagent,  and  when  the  resulting  solution  is  acidified  a 
reddish  brown  precipitate  is  formed.  Whether  this  precipitate 
contains  also  sulphides  of  arsenic,  antimony,  or  tin  is  easily 
determined  by  treating  the  precipitate  as  directed  on  page  44. 
To  detect  an  amount  of  copper  so  small  that  its  sulphide  would 
be  dissolved  entirely  by  the  ammonium  polysulphide,  a  solution 
may  be  used  that  has  been  obtained  by  boiling  with  HNO%  a 
small  portion  of  the  group  precipitate  before  it  has  been  treated 
with  the  polysulphide.  After  this  solution  has  been  evaporated 
to  small  bulk,  rendered  alkaline  by  the  addition  of  ammonium 
hydroxide,  and  then  acid  by  the  addition  of  acetic  acid,  it  may 
be  tested  for  copper  with  potassium  ferrocyanide. 


HYDROGEN   SULPHIDE  GROUP 


DIVISION   B 


FIRST   METHOD   OF  ANALYSIS 


To  the  precipitate  containing  only  sulphides  of  Division  B,  add  concentrated 
hydrochloric  acid  and  maintain  at  a  temperature  a  little  below  the  boiling 
point  for  some  minutes,  stirring  occasionally.  Filter. 


Residue:  Arsenic 

Boil  with  HCI  and  a 
crystal  of  KC103,  pro- 
longing the  operation 
until  the  residue  con- 
sists evidently  of  little 
else  than  free  sulphur. 
Filter  and  boil  the  fil- 
trate to  expel  chlorine. 

Portion  1.  Boil  with 
FeSO4  and  use  the  solu- 
tion for  (a)  Gatehouse 
test  (modified),  (b)  Bet- 
tendorff  test,  (c)Reinsch 
test. 

Portion2.  Add  a  few 
drops  of  the  solution  to 
5  cc.  of  (NH4)2Mo04  in 
a  small  test  tube  and 
warm  gently. 

Portion  3.  Make  al- 
kaline with  NH4OH  and 
add  MgCl2. 


Filtrate :  Tin,  Antimony,  (Arsenic) 

Warm  until  II2S  is  expelled. 

Portion  1.  Place  a  few  cubic  centimeters  of  the 
solution  in  a  porcelain  dish  and  introduce  a  piece  of 
platinum  foil  and  a  strip  of  zinc  in  such  a  way  that 
they  are  in  contact  with  each  other.  Dilute  with  a 
little  water  if  the  evolution  of  hydrogen  is  violent. 
Place  under  the  hood  and  allow  the  action  to  proceed 
until  the  evolution  of  hydrogen  has  nearly  ceased, 
adding  more  zinc  if  necessary.  Remove  the  zinc  and 
wash  it  with  a  stream  of  distilled  water  from  the 
wash  bottle.  With  the  finger  rub  off  into  a  porcelain 
dish  any  coating  that  may  still  adhere  to  the  zinc ; 
place  in  the  dish  a  clean  piece  of  platinum  foil ;  add 
HCI  and  boil  for  several  minutes.  Filter  if  a  residue 
remains.  Introduce  the  filtrate  into  a  small  test  tube 
that  has  been  rinsed  out  with  HgCl2,  and  boil.  A 
gray  precipitate  proves  the  presence  of  tin. 

If  the  precipitate  is  white,  pour  off  the  liquid  and 
treat  the  precipitate  with  NH4OH,  and  if  it  is  black- 
ened, the  presence  of  tin  is  proved. 

Portion  2.  To  a  few  drops  of  the  solution  on  a  plati- 
num foil  or  in  a  platinum  spoon  add  a  granule  of  zinc. 
A  black  stain  on  the  platinum  indicates  the  presence 
of  antimony.  Wash  the  foil  and  immerse  it  in  cold 
HCI  as  long  as  hydrogen  bubbles  appear ;  then  wash 
with  water.  Dissolve  the  stain  with  a  drop  of  fum- 
ing HN03,  add  a  little  crystallized  tartaric  acid,  and 
warm  gently  until  no  more  brown  fumes  appear. 
Dilute  with  two  or  tliree  volumes  of  water ;  add  a  few 
drops  of  HCI,  and  treat  with  H2S.  An  orange- 
colored  precipitate,  which  may  settle  only  after  the 
solution  has  stood  for  some  time,  proves  the  presence 
of  antimony. 

Portion  3.  Use  a  little  of  the  solution  for  the  modi- 
fied Bettendorff  test. 

Portion  4.  Test  a  few  drops  of  the  solution  directly 
for  arsenic  by  the  methods  given  under  Portion  1  in 
the  other  column,  in  case  no  reaction  for  arsenic  was 
obtained  with  the. portion  of  the  group  precipitate 
which  was  insoluble  in  cone,.  HCI  (Residue  :  Arsenic). 


ANALYSIS   OF   DIVISION   B  45 


DISCUSSION 

In  the  first  method  of  analysis  of  Division  B  arsenic  is 
separated  from  antimony  and  tin  by  taking  advantage  of 
the  difference  in  solubility  of  the  sulphides  in  hydrochloric 
acid,  the  sulphides  of  arsenic  being  but  slightly  attacked  while 
those  of  antimony  and  tin  are  dissolved  by  that  reagent. 

The  separation  is  not  altogether  sharp,  so  that  the  filtrate 
obtained  after  treatment  of  the  sulphides  with  hydrochloric 
acid  must  be  tested  for  arsenic  in  case  it  is  desired  to  detect 
mere  traces  of  that  element. 

The  detection  of  arsenic.  In  the  modified  Gutzeit  test  the 
nascent  hydrogen  is  produced  by  the  action  of  zinc  on  sul- 
phuric acid,  and  both  arsenious  and  arsenic  compounds  are 
reduced  to  arsine.  The  essential  point  of  the  test  is  the  for- 
mation of  the  yellow  compound  (AsAg3-  3AgNO3)  when  this 
gas  acts  upon  a  crystal  of  silver  nitrate.  The  disadvantages 
of  the  test  are  :  (a)  when  antimony  is  present  stibine  is  produced 
and  this  may  form  a  similar  yellow  compound  or  may  blacken 
the  silver  nitrate  crystal  at  once ;  (b)  if  hydrogen  sulphide  is 
evolved  from  the  reagents,  the  crystal  is  also  blackened. 
Although  the  latter  difficulty  may  be  avoided,  the  test  is  not 
so  simple  as  the  modified  Gatehouse  test. 

[The  Gutzeit  test  is  sometimes  applied  to  a  mixture  that 
may  contain  other  substances  than  compounds  of  arsenic,  anti- 
mony, and  tin.  When  this  is  done  it  must  be  borne  in  mind  that 
under  the  conditions  of  the  test : 

(a)  it  is  preferable  that  the  arsenic  in  the  substance  tested 
should  be  in  the  trivalent  condition; 

(b)  phosphites  and  hypophosphites  evolve   phosphine   which 
produces    exactly  the    same    color   changes  in  the   crystal  of 
AgNO3  as  does  arsenic;1 

1  See  Ber.  d.  chem.  Ges.,  16,  2435  (1883). 


46  HYDROGEN   SULPHIDE  GROUP 

(c)  sulphides l  and  sulphites,  if  present  in  large  amount,  will 
evolve  more  hydrogen  sulphide  than  can  be  absorbed  by  the  lead 
ammonium  acetate  in  the  cotton.] 

In  the  modified  Gatehouse  test  the  nascent  hydrogen  is  pro- 
duced by  the  action  of  aluminum  on  potassium  or  sodium 
hydroxide.  Only  arsenious  compounds  are  reduced.  It  has  an 
advantage  over  the  modified  Gutzeit  test,  since  (a)  no  stibine 
is  formed  when  antimony  is  present,  and  (b)  there  is  no  danger 
of  an  evolution  of  hydrogen  sulphide  from  the  reagents.  The 
action  in  the  tube  should  be  gentle;  otherwise  the  alkali  may 
be  carried  through  the  cotton  and  blacken  the  crystal. 

[If  in  this  test  there  is  used  a  mixture  that  may  contain  other 
elements  than  those  of  Division  B  of  the  hydrogen  sulphide 
group,  it  must  be  borne  in  mind  that  certain  gases  other  than 
arsine  produce  color  changes  in  a  crystal  of  silver  nitrate 
(see  above).] 

The  Bettendorff  test  is  delicate  when  concentrated  hydro- 
chloric acid  solutions  are  used.  Under  these  conditions  any 
arsenic  that  may  be  present  will  behave  like  arsenious  chloride 
and  will  be  reduced  by  stannous  chloride.  A  dilute  solution 
of  arsenious  acid  is,  however,  not  reduced  by  stannous 
chloride. 

The  Reinsch  test,  in  which  the  arsenic  is  deposited  upon  cop- 
per, is  not  in  itself  conclusive.  Deposits  may  also  be  formed  on 
the  copper  by  compounds  of  antimony  and  tin.  It  is  necessary 
therefore  to  supplement  the  Reinsch  test  by  other  tests.  If  on 
heating  the  copper  foil  in  a  glass  tube  a  sublimate  is  produced, 
and  this  then  responds  to  the  Gatehouse  test,  the  presence  of 
arsenic  is  proved. 

[If  elements  other  than  the  three  mentioned  are  present  in 

the  solution  submitted  to  the  Reinsch  test,  they  may  form  a 

deposit  on  the  copper.     Organic  matter  or  compounds  of  silver, 

mercury,  bismuth,  gold,  platinum,  palladium,  or  sulphur  may 

i  See  Ber.  d.  chem.  Ges.,  16,  2435  (1883). 


ANALYSIS  OF  DIVISION  B  47 

cause  such  deposits.  These  deposits,  however,  will  not  obscure 
the  test  for  arsenic  if  the  Reinsch  test  is  supplemented  as  above 
described.] 

Ammonium  molybdate  or  magnesium  chloride  cannot  be  used 
satisfactorily  in  the  detection  of  mere  traces  of  arsenic  acid. 
When  the  amount  of  arsenic  acid  is  small,  precipitation  takes 
place  so  slowly  that  the  rapid  and  accurate  tests  discussed 
above  are  to  be  preferred. 

The  detection  of  antimony.  In  the  modified  Gutzeit  test  if  the 
crystal  of  silver  nitrate  is  blackened  without  the  intermediate 
formation  of  a  yellow  compound,  the  presence  of  antimony  is 
indicated. 

In  the  modified  Bettendorff  test,  in  which  tin  foil  and  a 
moderately  dilute  solution  of  stannous  chloride  in  hydrochloric 
acid  form  the  reducing  agents,  the  presence  of  antimony  is  indi- 
cated if  the  tin  is  evenly  covered  with  a  black  deposit.  Arsenic, 
if  present,  will  be  deposited  more  unevenly  on  the  foil. 

A  similar  deposit  on  platinum  foil  when  a  granule  of  zinc  is 
placed  upon  the  platinum  in  the  acid  solution  to  be  tested  is 
also  an  indication  of  the  presence  of  antimony.  In  this  case  it 
is  possible  to  confirm  the  presence  of  this  element.  Any  metal- 
lic tin  which  may  also  have  been  deposited  on  the  platinum  will 
dissolve  easily  in  cold  HCI.  The  antimony  is  scarcely  attacked 
if  it  is  not  exposed  to  the  air  when  wet  with  the  acid. 

The  antimony  that  may  have  been  deposited  as  a  black  coat- 
ing on  the  platinum  is  instantly  changed  to  antimonic  acid  by 
the  action  of  fuming  nitric  acid.  Before  testing  for  antimony 
with  hydrogen  sulphide  the  excess  of  nitric  acid  is  removed  by 
heating  the  solution  with  crystallized  tartaric  acid.  This  is 
oxidized  by  the  nitric  acid,  which  is  itself  entirely  decom- 
posed. 

The  detection  of  tin.  When  platinum  and  zinc  are  placed  in  the 
acid  solution  of  a  tin  salt  there  is  formed  metallic  tin,  which  is 
chiefly  deposited  on  the  zinc.  When  this  tin  is  dissolved  in 


48  HYDROGEN  SULPHIDE  GROUP 

hydrochloric  acid  stannous  chloride  is  produced.  The  test  for 
tin  is  based  on  the  reducing  action  of  this  solution  on  mercuric 
chloride.  The  amount  of  mercuric  chloride  taken  is  small  in 
order  that  the  stannous  chloride  may  be  present  in  excess  and  a 
dark  precipitate  of  metallic  mercury  may  be  formed.  In  case 
the  amount  of  tin  is  so  small  that  the  reduction  does  not  pro- 
ceed beyond  the  formation  of  mercurous  chloride,  the  addition 
of  ammonium  hydroxide  and  the  consequent  formation  of  the 
black  mixture  of  mercury  and  Hg-NH2-Cl  will  show  that  the 
mercuric  chloride  has  been  reduced  and  that  therefore  tin  is 
present. 

SECOND   METHOD   OF   ANALYSIS 

Place  the  precipitate  in  an  evaporator ;  add  a  small  quantity 
of  concentrated  HC1  and  boil  the  liquid.  If  there  is  a  residue 
other  than  sulphur,  add  a  crystal  of  KC1O3  and  boil,  repeating 
the  operation  until  the  residue  consists  evidently  of  little 
but  free  sulphur.  Filter  and  reject  the  residue.  (If 
KC1O3  has  been  used,  boil  the  filtrate  until  the  odor  of 
chlorine  has  disappeared  ;  then  add  a  little  FeSO4  solution 
and  boil.) 

Pour  the  solution,  a  few  drops  at  a  time,  into  the  funnel  tube 
of  the  hydrogen  generator,  arranged  as  described  on  page  30, 
and  lead  the  evolved  gases  into  a  test  tube  half  full  of  AgNO3 
solution.  The  solution  should  be  added  so  slowly  that  the 
bubbles  of  gas  passing  through  the  solution  in  the  test  tube 
may  easily  be  counted.  The  zinc  should  be  kept  in  contact 
with  the  platinum  foil  during  the  whole  operation.  The 
action  should  be  allowed  to  continue  until  the  evolution  of 
gas  has  almost  ceased.  The  amount  of  zinc  that  is  used 
should  be  sufficient  to  insure  that  some  of  the  metal  still 
remains  in  the  generator  at  the  end  of  the  reaction. 


ANALYSIS  OF  DIVISION  B 


49 


Contents  of  the  Generator  :  Tin  and 
Antimony 


Filter  the  contents  of  the  generator,  re- 
jecting the  filtrate.     Remove  the  pieces  of 
zinc  and  the  platinum  foil,  and  wash  into 
the  filter  any  substance  loosely  adhering 
to  them.     Wash  the  contents  of  the  filter 
into  the  apex,  allow  to  drain,  and  tear  off 
the  apex  of  the  filter. 

If  a  black  precipitate  appears,  the 
presence  of  antimony  or  arsenic  is 
indicated. 
Filter. 

Filtrate:  Arsenic 

Add  HCI  until 
no  further  precip- 
itate   is    formed. 

Precipitate  : 
Antimony 

Wash  thorough- 
ly by  decantation 

Loose  Precipitate, 

Black  Stain  on 

chiefly  on  Zinc 

Platinum 

Shake  vigorously 

with  hot  water 

Place  the  apex  of 

If  the  platinum 

and  filter.     Pass 

until  the  wash 

the  filter  containing 

foil  is  stained  black, 

H2S  through  the 

water  gives  no 

the  precipitate,  to- 

antimony  is   proba- 

clear filtrate.     A 

turbidity    with 

gether  with  a  clean 

bly  present.  Immerse 

yellow  precipitate 

HCI.      Boil  the 

piece    of    platinum 

the  foil  in  cold  HCI 

proves  the  pres- 

precipitate with  a 

foil,  in  an  evapora- 

as long  as  hydrogen 

ence  of  arsenic. 

little  tartaric  acid. 

tor,  add  a  very  little 

bubbles  appear;  then 

To  confirm  the 

Filter.    Dilute  the 

HCI,    and    heat   to 

wash     with    water. 

presence  of  arse- 

filtrate with  a  little 

boiling  for   some 

Dissolve  the   stain 

nic,  especially  if 

water,  add  a  few 

minutes.     Filter, 

with  a  drop  of  fum- 

the precipitate  has 

drops  of  HCI,  and 

place  the  filtrate  in 

ing  HNO3,  add  a  little 

an  orange  tint, 

treat    with    H2S. 

a  small    test   tube 

crystallized   tartaric 

filter,  and  boil  the 

An  orange-col- 

that has  been  rinsed 

acid,  and  warm  gently 

precipitate    with 

ored    precipitate., 

out  with  HgCl2,  and 

until  no  more  brown 

a    little    HNOZ. 

which  may  appear 

boil.     A  white  pre- 

fumes appear.  Dilute 

Filter,  add  the 

only  after  the  solu- 

cipitate,   becoming 

with   two    or    three 

filtrate  to    5  cc. 

tion  has  stood  for 

gray  on   warming, 

volumes    of    water, 

(NH4)2MoO4,  and 

some  time,  proves 

proves  the  presence 

add  a  few  drops  of 

warm.     A   yel- 

the   presence    of 

of  tin.     If  the  pre- 

HCI, and  treat  with 

low    precipitate, 

antimony. 

cipitate  is  white, 

H2S.    An  orange- 

which    may    not 

pour  off  the  liquid 

colored    precipitate, 

appear    immedi- 

and treat  the  precip- 

which may  appear 

ately,     proves 

itate  with  NH4OH, 

only  after  the  solu- 

the   presence    of 

and  if  it  becomes 

tion   has   stood    for 

arsenic. 

blackened,  the  pres- 

some time,  proves  the 

ence  of  tin  is  proved. 

presence  of  antimony. 

Contents  of  the  Test  Tube 
Arsenic  and  Antimony 


50  HYBEOGEK   SULPHIDE   GKOUP 

DISCUSSION 

Of  the  sulphides  of  Division  B  of  the  hydrogen  sulphide 
group  which  are  precipitated  from  solutions  of  their  sulpho-salts 
by  dilute  hydrochloric  acid,  those  of  tin  and  antimony  are 
much  more  easily  soluble  in  hot  HCI  than  that  of  arsenic.  A 
suitable  oxidizing  agent,  however,  easily  changes  the  latter 
into  arsenic  acid,  and  this  is  soluble  in  water.  If,  therefore, 
the  addition  of  potassium  chlorate  is  needed  to  dissolve  the 
sulphides,  the  presence  of  arsenic  is  indicated. 

The  first  step  in  the  separation  of  the  members  of  Division  B 
is  the  reduction  by  nascent  hydrogen  of  a  solution  of  their 
compounds.  The  reaction  is  vigorous  but  obviously  not  instan- 
taneous. A  solution  that  has  been  prepared  as  above,  if  all 
the  members  of  this  division  are  present,  contains  stannic 
chloride,  antimonic  chloride,  and  arsenic  acid.  If  these  are 
first  reduced  to  stannous  chloride,  antimonious  chloride,  and 
arsenious  acid,  the  subsequent  reduction  in  the  generator  will  be 
completed  in  much  less  time.  This  is  especially  true  in  the  case 
of  arsenic  acid. 

The  solution  also  contains  the  oxidizing  agents  chlorine 
and  potassium  chlorate,  which  it  is  desirable  to  remove.  The 
chlorine  is  expelled  by  heating  the  solution,  the  potassium 
chlorate  being  reduced  at  the  same  time  if  a  sufficient  quan- 
tity of  hydrochloric  acid  is  present.  In  the  removal  of  these 
oxidizing  agents  the  antimonic  chloride  is  reduced  to  the 
trivalent  condition.  The  reduction  of  arsenic  acid  is  accom- 
plished by  the  addition  of  ferrous  sulphate.  The  stannic 
chloride  is  not  reduced  by  the  above  treatment,  but  this  is 
immaterial  since  its  reduction  is  easily  effected  by  the  nascent 
hydrogen. 

The  solution  should  be  introduced  into  the  generator  slowly 
and  in  small  portions : 


ANALYSIS  OF  DIVISION  B  51 

(a)  to  avoid  a  too  rapid  passage  of  the  evolved  gases  through 
the  silver  nitrate,  for  this  would  result  in  but  partial  interaction 
with  the  last-named  reagent ;  and 

(b)  to  avoid  the  presence,  at  any  one  time,  of  relatively  large 
amounts  of  the  arsenic  solution  in  the  generator,  for  this  might 
result  in  the  precipitation  of  metallic  arsenic  in  the  generator. 

The  separation  of  arsenic  and  antimony  from  tin  is  based  upon 
the  fact  that  nascent  hydrogen  reduces  compounds  of  tin  to 
metallic  tin,  while  compounds  of  arsenic  and  antimony  are 
reduced  to  arsine  and  stibine,  which  are  gases.  The  tin  remains 
in  the  generator,  and  the  two  gases  pass  off  with  the  hydrogen. 
Solid  arsenic  hydride  may  separate  in  the  generator  if  platinum 
foil  is  present,  but  under  ordinary  conditions  sufficient  arsine 
will  be  formed  to  make  the  detection  of  arsenic  possible  in  the 
gases  that  pass  out  of  the  generator.  As  tin  is  soluble  in  acids, 
it  is  necessary  to  allow  the  reaction  in  the  generator  to  proceed 
until  all  of  the  acid  is  decomposed  by  the  zinc. 

If  the  operation  is  carried  out  slowly,  as  directed,  the  reduc- 
tion of  compounds  of  antimony  to  stibine  is  seldom  complete,  a 
portion  of  the  antimony  being  deposited  in  the  metallic  form  as 
a  black  coating  upon  the  platinum.  Metallic  tin  also  may  be 
deposited  on  the  platinum.  In  such  case  most  of  it  is  but  loosely 
adherent  and  may  be  removed  by  washing.  The  remainder, 
which  in  the  absence  of  antimony  appears  as  a  gray  coating, 
will  easily  dissolve  in  cold  HCI  and  is  thus  separated  from  the 
antimony,  since  the  latter  is  but  slightly  soluble  in  the  cold 
acid. 

The  antimony  that  may  have  been  deposited  as  a  black  coat- 
ing on  the  platinum  is  instantly  changed  to  antimonic  acid  by 
the  action  of  fuming  nitric  acid.  Before  testing  for  antimony 
with  hydrogen  sulphide  the  excess  of  nitric  acid  is  removed 
by  heating  the  solution  with  crystallized  tartaric  acid.  This 
is  oxidized  by  the  nitric  acid,  which  is  itself  entirely 
decomposed. 


52  HYDBOGEN   SULPHIDE  GROUP 

The  separation  of  the  arsenic  and  the  antimony  that  have 
passed  out  of  the  generator  as  arsine  and  stibine  is  based  upon 
the  difference  in  the  action  of  those  gases  upon  a  solution  of 
silver  nitrate.  The  arsine  is  oxidized  to  arsenious  acid,  which 
remains  in  solution,  metallic  silver  being  at  the  same  time  pre- 
cipitated; while  the  stibine  forms  SbAg3,  which  is  also  pre- 
cipitated. The  formation  of  a  black  precipitate  in  the  silver 
nitrate,  therefore,  indicates  the  presence  of  arsenic  or  antimony. 
In  order  that  the  separation  may  be  as  complete  as  possible  it  is 
necessary  that  more  silver  nitrate  be  used  than  is  sufficient  to 
react  with  the  gases,  since  otherwise  an  excess  of  stibine  might 
dissolve  in  the  water  and  appear  in  the  filtrate  with  the  arseni- 
ous acid.  The  filtrate  from  the  precipitate  produced  by  the 
action  of  the  gases  on  silver  nitrate  will  therefore  contain  some 
unacted-upon  silver  nitrate  if  the  operation  has  been  properly 
carried  out.  The  silver  must  be  removed  by  precipitation  as 
silver  chloride  before  the  solution  is  tested  for  arsenic  with 
hydrogen  sulphide ;  otherwise  the  black  silver  sulphide  which 
would  be  formed  would  mask  the  color  of  the  yellow  arsenious 
sulphide. 

The  separation  of  arsenic  from  antimony  by  this  method  is 
not  altogether  sharp,  as  stibine  also  is  somewhat  oxidized  by 
silver  nitrate.  If  the  amount  of  stibine  is  relatively  large,  a 
sufficient  quantity  of  antimony  may  pass  into  solution  to  impart 
an  orange  color  to  the  precipitate  in  the  final  test  for  arsenic, 
even  if  an  excess  of  silver  nitrate  is  present  in  the  test  tube 
at  the  end  of  the  operation.  In  such  a  case  arsenic  may  be 
detected  by  boiling  the  precipitate  with  HNOy  The  sulphide 
of  arsenic  will  be  changed  to  arsenic  acid,  and  when  this  is  added 
to  ammonium  molybdate  a  yellow  precipitate  of  ammonium 
arseno-molybdate  is  formed. 


NICKEL  53 

AMMONIUM  SULPHIDE  GROUP 
PRELIMINARY  REACTIONS 

NICKEL  :  Reactions  of  solutions  of  nickel  salts. 

KOH  or  NaOH  precipitates  Ni(OH)2,  apple-green ;  insoluble  in  excess. 
Ni(OH)2  is  oxidized  by  boiling  with  bromine  water  and  NaOH  to  Ni(OH)3, 
black.  If  this  precipitate  is  filtered  out  and  boiled  with  NH4OH,  it  is 
reduced  to  Ni(OH)2,  nitrogen  being  at  the  same  time  liberated.  If  NH4C1 
is  present,  the  Ni(OH)2  which  is  formed  dissolves,  forming  perhaps 
NiCl2'4NH3. 

NH4OH,  if  added  in  relatively  small  quantity  to  a  solution  of  a  nickel 
salt,  precipitates  Ni(OH)2  ;  if  in  larger  quantity,  greenish  blue  basic  salts. 
The  precipitates  are  soluble  in  NH4OH  in  the  presence  of  ammonium  salts, 
forming  complex  salts,  as  Ni(N03)2'4  NH3.  These  solutions  and  precipi- 
tates are  changed  by  (NH4)2S  to  NiS,  black. 

Na2C03  precipitates  basic  carbonates,  apple-green;  soluble  in 
large  excess  of  the  concentrated  reagent. 

H2S  precipitates  NiS  from  an  ammoniacal  solution.  No  precipitate 
is  produced  in  solutions  containing  an  inorganic  acid  or  a  considerable 
quantity  of  acetic  acid. 

From  neutral  solutions  or  those  but  weakly  acidified  with 
acetic  acid,  prolonged  treatment  with  H2S  partially  precipitates 
the  nickel  as  NiS.  In  dilute  acetic  acid  solutions  containing 
an  alkali  acetate  the  precipitation  is  complete  and  fairly  rapid 
if  the  solution  is  warm. 

(NH4)2S  precipitates  from  neutral  or  alkaline  solutions  NiS,  black; 
somewhat  soluble  in  excess  of  (NH4)2S,  more  readily  in  the  presence  of 
NH4OH,  forming  a  dark  brown  solution  from  which  NiS  is  reprecipitated 
if  the  solution  is  boiled  and  the  solvent  is  thus  removed.  NiS  is  prac- 
tically insoluble  in  cold  dilute  HC1,  normal l ;  changed  by  aqua  regia  to  free 
sulphur  and  NiCl2  which  dissolves.  NiS  is  oxidized  by  the  air  to  NiS04. 

HC1  produces  no  precipitate. 

1  An  acid  of  this  strength  may  be  made  by  diluting  one  volume  of  the  dilute 
hydrochloric  acid,  which  is  twice  normal  (see  Appendix),  with  one  volume 
of  water.  It  may  also  be  prepared  by  diluting  one  volume  of  concentrated 
hydrochloric  acid  (specific  gravity  1.20)  with  twelve  volumes  of  water. 


54  AMMONIUM  SULPHIDE  GROUP 

H2S04  produces  no  precipitate. 

BaC03  when  shaken  with  a  cold  neutral  or  but  slightly  acid  solu- 
tion of  MC12  does  not  precipitate  a  compound  of  nickel.  BaCO3  is 
most  conveniently  used  for  this  purpose  in  a  state  of  suspen- 
sion in  water.  The  reagent  bottle  should  therefore  be  shaken 
before  using. 

A  Borax  Bead  is  colored  brown  when  fused  with  a  compound  of  nickel. 
Heat  a  small  loop  of  clean  platinum  wire  to  redness  and  dip  it  into  pow- 
dered borax.  Heat  gently  at  first  and  then  fuse  until  all  bubbles  dis- 
appear and  a  clear  bead  results.  Touch  the  solid  substance  to  be  tested 
with  the  hot  bead  in  such  a  way  that  a  small  amount  adheres  to  the  bead 
and  fuse  again  until  clear  and  free  from  bubbles.  Note  the  color. 

COBALT  :  Reactions  of  solutions  of  cobalt  salts. 

KOH  or  NaOH  precipitates  from  cold  solutions  a  blue  basic  salt  which 
when  warmed  with  the  alkali  changes  to  Co(OH)2,  pink.  Co(OH)2  may 
be  oxidized  to  Co(OH)3,  black.  The  reaction  takes  place  slowly  in  the  air, 
more  readily  under  the  influence  of  oxidizing  agents,  such  as  bromine  water 
in  the  presence  of  an  alkali  (NaOH).  Co(OH)3  is  not  reduced  by  boiling 
it  with  NH4OH. 

NH4OH  precipitates  blue  basic  salts ;  easily  soluble  in  NH4OH  in  the 
presence  of  ammonium  salts,  forming  complex  salts,  as  Co(N03)2'4NH3. 
This  solution  quickly  assumes  a  red  color,  being  oxidized  by  the  air  to  com- 
plex cobaltic  ammonia  compounds.  These  solutions  and  precipitates  are 
changed  by  (NH4)2S  to  CoS,  black. 

Na2C03  precipitates  pink  basic  carbonates;  soluble  in  large 
excess  of  the  concentrated  reagent. 

H2S.  The  action  of  this  reagent  upon  solutions  of  cobalt  salts 
is  similar  to  that  upon  solutions  of  nickel  salts. 

(NH4)2S  precipitates  from  neutral  or  alkaline  solutions  CoS,  black ; 
insoluble  in  excess ;  practically  insoluble  in  cold  normal l  HC1 ;  attacked 
by  aqua  regia,  forming  sulphur  and  CoCl2  which  dissolves.  CoS  is  oxi- 
dized by  the  air  to  CoS04. 

HC1  produces  no  precipitate. 

H2S04  produces  no  precipitate. 

1  See  note,  page  53. 


IRON  55 

BaC03  when  shaken  with  a  cold  neutral  or  but  slightly  acid 
solution  of  CoCl2  does  not  precipitate  a  compound  of  cobalt. 
A  Borax  Bead  is  colored  blue  when  fused  with  a  compound  of  cobalt. 

IRON  :  Reactions  of  solutions  of  ferrous  salts. 

KOH  or  NaOH  precipitates  Fe(OH)2,  white,  which  is  immediately  oxi- 
dized to  a  dirty  green  compound  unless  special  precautions  are  taken. 
The  oxidation  proceeds  rapidly  when  the  precipitate  is  exposed  to  the  air, 
until  finally  it  is  transformed  into  Fe(OH)3,  reddish  brown.  Immediate 
precipitation  is  prevented  by  the  presence  of  a  relatively  large 
quantity  of  ammonium  salts. 

NH4OH  produces  in  neutral  solutions  an  incomplete  precipitation  of 
Fe(OH)2,  a  portion  of  the  iron  remaining  in  the  solution  as  a  double  salt, 
such  as  FeS04-(NH4)2S04  or  FeCl^  NH4C1.  The  precipitate  passes 
through  the  different  stages  of  oxidation  described  in  the  preceding  para- 
graph. In  solutions  containing  double  salts  like  those  above  mentioned, 
NH4OH  produces  no  precipitate  immediately,  but  oxidation  by  the  air 
gradually  takes  place  and  there  results  a  precipitate  which  is  at  first 
green,  then  black,  and  finally  reddish  brown.  When  these  solutions  or 
precipitates  are  treated  with  (NH4)2S  there  is  formed  FeS,  black. 

N^COg  precipitates  FeCO3,  white,  which  is  quickly  oxidized 
by  the  air  to  a  green  compound  and  finally  to  Fe(OH)3.  FeCO3 
is  soluble  in  large  excess  ,of  a  concentrated  solution  of  Na2CO3. 

I^S  produces  no  precipitate  in  solutions  containing  dilute 
inorganic  acids.  From  dilute  neutral  solutions  a  slight  precipi- 
tate of  FeS  may  be  obtained  by  prolonged  treatment  with  H2S. 
In  the  presence  of  a  large  amount  of  an  alkali  acetate  there  is 
a  considerable  precipitation  of  FeS,  but  this  is  not  complete. 

(NH4)2S  precipitates  .FeS,  black ;  attacked  by  hot  HNO%,  forming  sul- 
phur and  Fe(N03)3  which  dissolves.  FeS  is  soluble  in  a  large  amount  of 
cold  dilute  HC1.  FeS  on  exposure  to  moist  air  is  oxidized  to  FeS04  and 
finally  to  a  basic  ferric  sulphate  which  is  brown. 

HC1  produces  no  precipitate. 

H2S04  produces  no  precipitate. 

BaC03  shaken  with  a  cold  neutral  or  but  slightly  acid  solution  of  FeC^ 
does  not  precipitate  a  compound  of  iron.  BaC03  is  most  conveniently 


56  AMMONIUM  SULPHIDE  GKOUP 

used  for  this  purpose  in  a  state  of  suspension  in  water.  The  reagent 
bottle  should  therefore  be  shaken  before  using. 

A  solution  of  a  ferrous  salt  always  contains  some  ferric  salt  unless 
extraordinary  precautions  have  been  taken  to  prevent  oxidation.  For  the 
first  two  of  the  following  tests  it  is  necessary  to  prepare  a  fresh  solution 
of  a  ferrous  salt.  This  may  be  done  by  adding  dilute  H2S04  and  a  few 
iron  filings  to  a  solution  of  ferrous  sulphate  and  warming  until  the  reac- 
tion is  well  started.  This  will  quickly  reduce  to  the  ferrous  condition 
any  ferric  salt  that  may  be  present.  The  operation  may  be  carried  out  in 
a  small  Erlenmeyer  flask,  fitted  with  a  cork  through  which  passes  a  glass 
tube  drawn  out  to  small  diameter  at  the  outer  end.  It  should  be  borne 
in  mind  that  the  mixture  of  gases  in  the  flask  is  explosive. 

KCNS,  potassium  sulphocyanate,  produces  no  coloration  in  solutions  of 
ferrous  salts. 

K4Fe(CN)6,  potassium  ferrocyanide,  precipitates  K2Fe"Fe(CN)6, 
potassium  ferrous  ferrocyanide.  This  compound  is  white,  but  is 
so  easily  oxidized  that  under  ordinary  conditions  of  preparation 
it  has  a  light  blue  color,  due  to  the  formation  of  a  small  amount 
of  Fe;"[Fe(CN)8]s. 

K3Fe(CN)6,  potassium  ferricyanide,  precipitates  from  neutral  or 
acid  solutions  Fe3[Fe(CN)6]2,  dark  blue;  insoluble  in  dilute  acids. 

HN03.  If  a  few  drops  of  HNO%  are  added  to  a  solution  of  a  ferrous 
salt  and  the  mixture  is  boiled,  a  ferric  salt  will  be  formed. 

Chlorine  water  oxidizes  a  solution  of  a  ferrous  salt  to  a  ferric  salt. 
The  chlorine  for  this  oxidation  may  be  obtained  by  adding  a  small  crystal 
of  KC103  to  a  solution  of  a  ferrous  salt  containing  HCI  and  then  boiling 
the  mixture. 

Reactions  of  solutions  of  ferric  salts. 

KOH  or  NaOH  precipitates  Fe(OH)3,  reddish  brown,  gelatinous ; 
transformed  by  (NH4)2S  into  FeS. 

NH4OH  precipitates  Fe(OH)3.  The  presence  of  non-volatile 
organic  acids  tends  to  prevent  precipitation. 

Na2C03  precipitates  a  reddish  brown  basic  carbonate  which 
changes  to  Fe(OH)3  when  boiled  with  the  reagent.  The  basic 
carbonate  is  soluble  in  a  large  excess  of  the  concentrated 
reagent. 


IEON  5T 

H2S  reduces  solutions  of  ferric  salts  containing  HC1  or  H2S04  to  the 
ferrous  condition,  sulphur  being  set  free. 

(NH4)2S  precipitates  FeS  mixed  with  free  sulphur. 

HC1  produces  no  precipitate. 

H2S04  produces  no  precipitate. 

BaC03  when  shaken  with  a  cold  neutral  or  but  slightly  acid  solution 
of  a  ferric  salt  precipitates  Fe(OH)3  or  a  basic  salt.1 

KCNS  when  added  in  excess  to  a  solution  of  a  ferric  salt  forms 
Fe(CNS)3-9KCNS,  a  deep  red  salt2  which  is  soluble  in  water  and  there- 
fore imparts  its  color  to  the  solution.  The  color  is  most  intense  when 
the  solution  contains  a  small  amount  of  a  free  inorganic  acid.  Solu- 
tions containing  oxides  of  nitrogen  and  a  free  inorganic  acid  also  give  a 
red  color  when  KCNS  is  added.  Under  certain  conditions  the  forma- 
tion of  Fe(CNS)3-9  KCNS  and  the  consequent  red  coloration 
of  the  solution  may  not  result.  A  ferric  solution  that  contains 
an  excess  of  an  alkali  acetate  (a  ferric  acetate  solution)  will  give 
no  red  color  with  potassium  sulphocyanate  until  a  considerable 
amount  of  an  inorganic  acid,  such  as  hydrochloric  acid,  has  been 
added.  This  is  also  true  of  ferric  solutions  containing  phos- 
phoric, oxalic,  tartaric,  or  boric  acid.  When  ether  is  added  to 
a  solution  containing  ferric  sulphocyanate  and  the  mixture  is 
shaken,  the  ferric  sulphocyanate  is  taken  up  by  the  ether  and 
imparts  to  the  ether  layer  a  red  color.  This  fact  may  be  utilized 
in  testing  for  traces  of  iron  so  minute  that  the  red  coloration 
with  KCNS  is  scarcely  discernible. 

K4Fe(CN)6  precipitates  Fe'J'[Fe(CN)6]8,  dark  blue ;  not  readily 
soluble  in  dilute  inorganic  acids.  If  the  solutions  are  dilute,  the 
liquid  may  assume  a  deep  blue  color  without  the  formation  of  a 
precipitate. 

K3Fe(CN)6  produces  no  precipitate  but  imparts  a  brown  color 
to  the  solution. 

Most  reducing  agents  easily  reduce  ferric  to  ferrous  salts. 

1  Dammer  in  his  Handbuch  der  anorganischen  Chemie  states  that  the  precipi- 
tate is  ferric  hydroxide.     But  Demarc.ay,  the  authority  to  whom  he  refers,  says : 
"  Iron  is  precipitated  by  barium  carbonate  always  in  the  form  of  a  basic  salt," 

2  Krliss  and  Moraht:  Ber.  d.  chem.  Ges.,  22,  2061  (1889). 


58  AMMONIUM  SULPHIDE  GROUP 

MANGANESE  :   Reactions  of  solutions  of  manganous  salts. 

KOH«or  NaOH  precipitates  Mn(OH)2,  white;  oxidized  quickly 
by  the  air,  probably  forming  first  manganous  acid,  MnO(OH)2,1 
which  then  reacts  with  Mn(OH)2  to  form  manganous  manganite, 
Mn2O3.  These  compounds  are  dark  brown. 

NH4OH,  in  the  absence  of  ammonium  salts,  precipitates  one-half  of  the 
manganese  as  Mn(OH)2  (see  preceding  paragraph),  the  other  half  remain- 
ing in  solution  as  a  double  salt,  e.g.,  MnCl2-2NH4Cl  or  MnS04-(NH4)2S04. 
In  solutions  of  such  double  salts,  NH4OH  produces  no  immediate  pre- 
cipitate, but  the  solution  soon  begins  to  be  oxidized  by  the  oxygen  of 
the  air,  and  MnO(OH)2  is  precipitated.  These  solutions  and  precipitates 
are  changed  to  MnS  by  (NH4)2S. 

Na^Og  precipitates  MnCO3  or  basic  carbonates,  according  to 
the  conditions  under  which  precipitation  takes  place;  white, 
oxidizing  in  the  air  to  MnO(OH)2. 

H2S  produces  no  precipitate  from  solutions  containing  inorganic  acids  or 
a  moderate  amount  of  acetic  acid.  From  a  solution  of  Mn(C2H3O2)2 
containing  but  little  acetic  acid,  prolonged  treatment  with  H2S 
may  precipitate  a  small  quantity  of  MnS,  pink. 

(NH4)2S  precipitates  MnS,  pink;  soluble  in  dilute  inorganic  acids  or 
acetic  acid.  MnS  turns  brown  upon  oxidation  in  the  air,  with  the 
formation  of  Mn203,  MnS04,  and  sulphur.  Oxalates,  tartrates,  or 
citrates  tend  to  prevent  precipitation.  By  boiling  the  pink 
sulphide  with  a  large  excess  of  (NH4)2S  and  NH4OH  it  may 
be  changed  into  another  sulphide,2  which  is  green,  more  com- 
pact, less  easily  oxidized,  and  less  easily  soluble  in  acetic  acid 
than  the  pink  sulphide. 

HC1  produces  no  precipitate. 

H2S04  produces  no  precipitate. 

BaC03  when  shaken  with  a  cold  neutral  or  but  slightly  acid  solution  of 
MnCL,  does  not  precipitate  a  compound  of  manganese. 

Fusion  with  N^COg  and  KN03  oxidizes  manganese  compounds, 


1  Tread  well  :  Qualitative  Analyse,  p.  102. 

2  Meineke  :  Ztschr.  angew.  Chem.,  1888,  p.  3. 


ZINC  59 

in  which  the  valence  of  manganese  is  less  than  six,  to  K2MnO4 
and  Na2MnO4,  green. 

Use  a  relatively  small  quantity  of  the  dry  substance  to  be 
tested  and  carry  out  the  fusion  in  a  platinum  spoon.  If  the 
test  is  properly  performed,  the  color  is  best  seen  at  the  edges 
of  the  fused  mass. 

Pb02,  when  boiled  with  dilute  H2S04  and  a  small  quantity  of  a  manga- 
nese compound  in  which  the  valence  of  manganese  is  less  than  seven,  oxi- 
dizes the  latter  to  HMn04  which  imparts  a  pink  or  purple  color  to  the 
solution  according  to  the  amount  of  manganese  present.  The  color  is  best 
seen  after  the  PbS04  and  the  excess  of  Pb02  have  been  allowed  to  settle. 
If  a  chloride  or  HC1  is  present  in  any  considerable  quantity,  it  should 
first  be  removed  by  adding  concentrated  H2S04  to  the  dry  substance  to 
be  tested  and  heating  until  dense  white  fumes  of  S03  appear. 

ZINC  :  Reactions  of  solutions  of  zinc  salts. 

KOH  or  NaOH  precipitates  Zn(OH)2,  white,  gelatinous ;  soluble 
in  an  excess  of  either  reagent,  forming  salts  such  as  potassium 
zincate,  Zn(OK)2. 

NH4OH  produces  in  neutral  solutions  a  partial  precipitation  of  Zn(OH)2, 
white,  gelatinous ;  soluble  in  NH4OH  and  NH4C1,  forming  ZnCl2'4NH3. 
Zinc  hydroxide  is  soluble  in  ammonium  salts,  forming  double  salts  :  with 
NH4C1,  for  example,  there  is  formed  ZnCl2-2NH4Cl.  For  this  reason  no 
precipitate  is  produced  by  the  addition  of  NH4OH  to  a  solution  containing 
a  relatively  large  amount  of  an  ammonium  salt.  When  any  of  these 
solutions  or  Zn(OH)2  is  treated  with  (NH4)2S,  ZnS  is  formed. 

Na^Og  precipitates  white  basic  carbonates;  soluble  in  large 
excess  of  the  concentrated  reagent. 

H2S  precipitates  ZnS  incompletely  from  neutral  solutions  of  zinc  salts 
of  the  inorganic  acids.  ZnS  is  white  and,  when  freshly  precipitated,  is 
soluble  in  dilute  inorganic  acids.  ZnS  is  only  slightly  soluble  in  acetic  acid. 
The  precipitation  is  complete  if  an  alkali  acetate  (NaC2H3O2) 
is  present.1  From  alkaline  solutions  H2S  precipitates  ZnS 
completely. 

1  For  full  discussion  of  this  point  see  Introduction,  page  4. 


60  AMMONIUM  SULPHIDE  GliOUP 

(NH4)2S  precipitates  ZnS. 

HC1  produces  no  precipitate. 

H2S04  produces  no  precipitate. 

BaC03  when  shaken  with  a  cold  neutral  or  but  slightly  acid  solution  of 
ZnCLj  does  not  precipitate  a  compound  of  zinc. 

I 

ALUMINUM  :  Reactions  of  solutions  of  aluminum  salts. 

KOH  or  NaOH  precipitates  A1(OH)3,  white,  gelatinous ;  A1(OH)3  is 
easily  soluble  in  an  excess  of  either  reagent,  forming  an  alkali  alumi- 
nate  such  as  Al(ONa)3.  No  precipitate  is  formed  when  this  solution  is 
boiled  alone,  but  if  NH4C1  is  added  before  boiling,  A1(OH)3  is  pre- 
cipitated. A1(OH)3  is  easily  soluble  in  dilute  inorganic  acids  when  freshly 
precipitated. 

NH4OH  precipitates  A1(OH)3,  soluble  with  difficulty  in  NH4OH,  probably 
forming  A1(ONH4)3.  A1(OH)3  is  reprecipitated  when  this  solution  is 
gently  warmed. 

vxNa2C03  precipitates  A1(OH)3  or  basic  carbonates. 

I^S  produces  no  precipitate  from  neutral  or  acid  solutions  of 
aluminum  salts.  A1(OH)3  will  be  precipitated  by  H2S  from  a 
solution  of  A1(OK)3,  but  this  precipitate  dissolves  if  more  KOH 
is  added. 

v/  (NH4)2S  precipitates  A1(OH)3  from  aqueous  solutions  of  aluminum 
salts.  The  hydroxide  and  not  the  sulphide  is  formed,  because  the  sul- 
phide cannot  exist  in  the  presence  of  water. 

^  HC1  produces  no  precipitate. 
v/H2S04  produces  no  precipitate. 

^  BaC03  when  shaken  with  a  cold  neutral  or  but  slightly  acid  solution  of 
A1C13  precipitates  A1(OH)3  or  basic  carbonates. 

CHROMIUM  :  Reactions  of  solutions  of  chromium  salts. 

KOH  or  NaOH  precipitates  Cr(OH)3,  grayish  green  or  grayish  lavender, 
gelatinous ;  easily  soluble  in  an  excess  of  the  reagent,  forming  Cr(OK)3 
or  Cr(ONa)3,  which  imparts  a  green  color  to  the  solution.  When  this 
solution  is  boiled,  Cr(OH)3  is  reprecipitated.  The  reprecipitation  takes 

/ 


CHKOMIUM  61 

place  more  readily  when  the  solution  is  dilute  or  an  ammonium  salt  is 
present.      Cr(OH)3  is  soluble  in  dilute  inorganic  acids. 

NH4OH  precipitates  Cr(OH)3  ;  soluble  with  difficulty  in  NH4OH  in  the 
presence  of  ammonium  chloride,  forming  a  red  solution  containing  chiefly 
CrCl3'4  NHg.1  When  this  solution  is  boiled,  Cr(OH)3  is  reprecipitated. 

-    Na^Og  precipitates  Cr(OH)3  or  basic  carbonates;  soluble  in 
large  excess  of  the  concentrated  reagent. 

H2S  produces  no  precipitate  in  an  acid  solution  of  a  chromium 
salt. 
^(NH4)2S  precipitates  Cr(OH)3. 

HC1  produces  no  precipitate. 

H2S04  produces  no  precipitate. 

BaC03  when  shaken  with  a  cold  neutral  or  but  slightly 
acid  solution  of  CrCl3  precipitates  Cr(OH)3  or  a  basic  car- 
bonate. 

Fusion  with  KN03  and  Na2C03  oxidizes  chromium  compounds, 
in  which  the  valence  of  chromium  is  less  than  six,  to  K2CrO4 
and  Na2CrO4,  yellow.  If  the  fused  mass  is  dissolved  in  water 
and  the  solution  acidified  with  acetic  acid,  the  addition  of  lead 
ammonium  acetate  precipitates  PbCrO4,  yellow;  soluble  in 
NaOH,  forming  Pb(ONa)2. 

H202  oxidizes  a  hot  solution  of  Cr(OK)3  or  Cr(ONa)3  to  JLfrO^  or 
Na^rO^  When  H2S04  is  present  H202  oxidizes  a  chromate  probably 
to  perchromic  acid,  HCr04,  which  imparts  a  blue  color  to  the  liquid. 
An  excess  of  H202  reduces  the  perchromic  acid  to  chromium  sulphate, 
the  tendency  to  reduction  increasing  with  the  amount  of  free  acid 
present. 

When  ether  is  added  to  the  above-mentioned  blue  solution  of 
perchromic  acid  (?)  and  the  mixture  is  shaken,  the  blue  sub- 
stance is  taken  up  by  the  ether,  and  imparts  its  color  to  the 
ether  layer. 

1  Dammer,  Handbuch  der  anorganischen  Chemie,  III,  p.  552. 


62  AMMONIUM  SULPHIDE  GROUP 

METHOD  OF  ANALYSTS 

(NOTE.  —  The  portions  of  the  text  that  are  enclosed  in  heavy  brackets  deal  with 
the  analysis  of  the  ammonium  sulphide  group  when  oxalates,  tartrates,  and  phosphates 
are  present.  The  study  of  this  matter  should  be  deferred  until  the  student  has  famil- 
iarized himself  with  the  procedure  that  is  followed  when  the  above-named  salts  are 
absent.) 

Unless  oxalates,  tartrates,  and  phosphates  are  known  to  be  absent,  they 
must  be  tested  for  before  the  analysis  of  this  group  is  begun. 

If  oxalates  or  tartrates  are  present,  carefully  evaporate 
the  filtrate  from  the  hydrogen  sulphide  group  just  to  dry- 
ness.  Grind  the  dry  residue  with  five  times  its  quantity  of 
solid  NH4NO3.  In  a  large  porcelain  crucible  fuse  enough 
NH4NO3  to  cover  the  bottom  and  introduce  the  above  mix- 
ture in  very  small  portions.  Maintain  the  contents  of  the 
crucible  in  gentle  fusion  during  the  addition  and  for  some 
time  thereafter.  When  cool,  dissolve  the  contents  of  the 
cible  in  water,  or  HC1  if  necessary,  and  treat  the  solu- 
ected  in  the  next  paragraph. 

If  oxalates  or  tartrates  are  absent^  add  NH4C1  to  the  filtrate 
from  the  hydrogen  sulphide  group  or  to  the  solution  known  to 
contain  only  members  of  the  (NH4)2S  group,  and  divide  into 
two  portions. 

PORTION  1 

To  a  small  portion  of  the  solution,  freed  from-H2S  by  boiling 
if  the  filtrate  from  the  H2S  group  is  used,  add  NH4OH  till 
slightly  alkaline,  and  note  whether  a  precipitate  forms.  If  it 
does,  then  add  NH4OH  quickly  in  large  excess  and  observe 
whether  the  precipitate  redissolves.  Note  the  colors  of  both 
precipitate  and  solution  and  any  changes  in  color  that  may 
occur  during  the  operation,  and  record  the  inferences  that  may 
be  drawn  from  these  observations. 


ANALYSIS  63 

PORTION  2 

Place  5  cc.  of  this  portion  in  a  test  tube,  add  NH4OH  until 
the  solution  is  very  slightly  alkaline,  heat  to  boiling,  and  then 
add  a  few  drops  of  (NH4)2S.  If  no  precipitate  appears  even 
after  the  addition  of  (NH4)2S,  analyze  the  remainder  of  Portion  2 
as  directed  on  page  75  unless  members  of  succeeding  groups  are 
known  to  be  absent. 

If  a  precipitate  has  appeared,  discard  the  contents  of  the 
test  tube  and  to  the  remainder  of  Portion  2  add  NH4OH  until 
the  solution  is  very  slightly  alkaline.  Heat  to  boiling.  Add 
(NH4)2S  in  slight  excess,  heat  again  to  boiling,  and  allow  the 
precipitate  to  settle.  Filter  and  discard  the  nitrate  if  it  is 
known  that  members  of  succeeding  groups  are  absent ;  other- 
wise treat  the  filtrate  as  directed  on  page  75.  If  this  nitrate 
is  dark  brown  in  color,  the  presence  of  nickel  is  indicated.1 
Wash  the  precipitate  immediately  and  thoroughly  with  hot 
water  to  which  a  few  drops  of  NH4C1  and  (NH4)2S  have  been 
added,  and  without  delay  transfer  the  precipitate  to  an  evap- 
orator and  treat  it  with  cold  normal2  HC1  for  a  few  minutes. 
Stir  the  mixture  vigorously  to  bring  all  parts  of  the  precipitate 
into  contact  with  the  acid.  If  a  black  residue  remains,  the 
presence  of  nickel  or  cobalt  is  indicated.  Filter  and  wash 
the  residue  immediately.  (If  the  residue  is  light  colored,  not 
otherwise,  it  may  be  dissolved  by  warming  with  HC1.) 

1  This  filtrate  should  not  have  a  brown  color,  and  will  not  be  so  colored  if 
the  precipitation  of  the  group  has  properly  been  performed. 

2  See  note,  page  53. 


64 


AMMONIUM  SULPHIDE  GROUP 


ANALYSIS 


65 


2<5  So 

III- 


66  AMMONIUM   SULPHIDE    GBOUP 

DISCUSSION 

—  — 

If  the  filtrate  from  the  hydrogen  sulphide  group  contains 

oxalates  or  tartrates  they  must  be  removed,  since  in  their 
presence  some  of  the  members  of  the  ammonium  sulphide 
group  are  precipitated  only  partially  if  at  all.  Moreover,  if 
oxalates  and  at  the  same  time  members  of  the  ammonium 
carbonate  group  are  present,  the  latter  will  be  precipitated 
as  oxalates  in  the  ammonium  sulphide  group  and  might  thus 
escape  detection.  Fusion  with  ammonium  nitrate  oxidizes 
and  destroys  these  and  other  organic  compounds  that  might 
exert  a  disturbing  influence. 

—  — 

The  separation  of  the  members  of  the  ammonium  sulphide 
group  from  those  of  the  following  groups  is  based  upon  the 
insolubility  in  water  of  the  hydroxides  of  aluminum  and  chro- 
mium and  of  the  sulphides  of  the  other  members  of  the  group. 
The  hydroxides  and  sulphides  of  the  alkali  metals  are  soluble 
in  water.  The  sulphides  of  the  members  of  the  ammonium 
carbonate  group  and  of  magnesium  are  not  formed  in  the 
presence  of  water,  but  their  hydroxides  are  produced  by  the 
action  of  ammonium  hydroxide.  These  hydroxides,  except  that 
of  magnesium,  are  soluble  in  water.  The  addition  of  ammonium 
chloride  prevents  the  precipitation  of  magnesium  hydroxide, 
there  being  formed  the  double  salt  MgCl2-  2  NH4C1,  which  is 
soluble  in  water  and  unaffected  by  ammonium  hydroxide. 

The  presence  of  ammonium  chloride,  moreover,  prevents  the 
formation  of  pseudo-solutions  and  causes  the  members  of  the 
ammonium  sulphide  group  to  be  precipitated  in  a  more  compact 
form.  It  furthermore  prevents  the  complete  precipitation  of 
borates  of  the  ammonium  carbonate  group. 

The  test  with  ammonium  hydroxide  often  furnishes  indica- 
tions of  value  and  sometimes  conclusive  proof  as  to  the  presence 
or  absence  of  certain  members  of  the  group.  As  ammonium 
sulphide  is  formed  by  the  action  of  ammonium  hydroxide  on 


ANALYSIS  67 

hydrogen  sulphide,  the  latter  must  be  expelled  from  the  portion 
of  the  solution  in  which  this  test  is  made. 

In  the  precipitation  of  the  group,  ammonium  hydroxide  is 
added  to  neutralize  any  free  acid  which,  if  present,  would 
decompose  the  ammonium  sulphide.  An  excess  of  ammonium 
hydroxide  is  to  be  avoided,  both  on  account  of  the  solubility  of 
the  hydroxides  of  aluminum  and  chromium  in  that  reagent,  and 
also  because  its  presence  in  large  amount  increases  the  solubility 
of  nickel  sulphide  in  ammonium  sulphide,  thus  rendering  nitra- 
tion slow  and  increasing  the  danger  of  oxidation. 

The  group  precipitate  is  filtered  rapidly  and  immediately 
after  precipitation,  and  is  washed  with  water  containing  ammo- 
nium sulphide,  in  order  to  prevent  the,  oxidation  of  the  sulphides 
of  nickel  and  cobalt  to  the  sulphates  which  would  dissolve 
when  the  group  precipitate  is  treated  with  dilute  hydrochloric 
acid.  Ammonium  chloride  is  aocMi^ to  the  wash  water  to 
prevent  the  precipitate  from  dissolving  in  water  and  forming  a 
pseudo-solution. 

The  separation  of  nickel  and  cobalt  from  the  other  members 
of  the  group  is  based  upon  the  fact  that  the  sulphides  of  nickel 
and  cobalt  are  but  very  slightly  soluble  in  normal  hydrochloric 
acid,  while  the  sulphides  of  iron,  zinc,  and  manganese  and  the 
hydroxides  of  aluminum  and  chromium  are  more  readily  dis- 
solved by  hydrochloric  acid  of  this  strength.  A  black  residue 
after  this  treatment  indicates,  but  does  not  prove,  the  presence 
of  nickel  or  cobalt,  for  some  of  the  ferrous  sulphide  is  at  times 
left  undissolved. 

Other  members  of  this  group  may  also  remain  with  this 
residue  even  if  a  reasonable  amount  of  hydrochloric  acid  has 
been  used,  for  some  of  the  sulphides  and  hydroxides  that  are 
dissolved  by  acids  when  freshly  precipitated  are  much  less 
readily  dissolved  some  time  after  their  formation.  If,  however, 
sufficient  acid  is  used,  enough  of  these  elements  will  pass  into 
the  filtrate  to  be  detected  in  the  subsequent  tests. 


68  AMMONIUM  SULPHIDE  GROUP 

The  separation  of  nickel  from  cobalt  is  based  upon  the  facts, 
(1)  that  when  their  trihydroxides  are  boiled  with  ammonium 
hydroxide  that  of  cobalt  is  unchanged,  while  that  of  nickel  is 
reduced  to  Ni(OH)2;  and  (2)  that  Ni(OH)2  is  dissolved  by 
ammonium  hydroxide  and  ammonium  chloride,  while  Co(OH)3  is 
insoluble.  If  in  the  oxidation  to  the  trihydroxides  any  Co(OH)2 
is  left  unoxidized,  this  also  will  dissolve  in  ammonium  hydroxide 
and  ammonium  chloride  and  will  be  precipitated  from  this  solu- 
tion by  hydrogen  sulphide ;  hence  the  necessity  of  fusing  with 
the  borax  bead  the  precipitate  obtained  in  the  final  test  for  nickel. 

After  the  removal  of  the  sulphides  of  nickel  and  cobalt  by 
filtration,  hydrogen  sulphide  is  expelled  from  the  filtrate  to  pre- 
vent the  formation  of  free  sulphur  in  the  subsequent  oxida- 
tion, of  ferrous  to  ferric  salts.  This  oxidation  is  necessary  (1) 
because  the  tests  for  ferric  iron  (KCXS,  etc.)  are  more  char- 
acteristic and  delicate  tnan  those  for  ferrous  iron,  and  (2) 
because  iron  is  completely  precipitated  by  barium  carbonate 
only  when  it  is  in  the  form  of  a  ferric  salt. 

Ferrous  salts  in  dilute  hydrochloric  acid  solution  are  easily 
oxidized  to  ferric  salts  by  boiling  with  a  little  HNOZ,  and 
this  oxidizing  agent  is  used  in  the  preparation  of  s  the  portion 
of  the  solution  that  is  to  be  treated  with  bai«ftim  carbonate. 
Since  nitric  acid  may  itself  give  a  pink  color  with  potassium 
sulphocyanate,  a  smaller  portion  of  the  dilute  hydrochloric  acid 
solution  is  oxidized  by  chlorine  water  instead  of  nitric  acid,  and 
this  solution,  after  the  removal  of  the  excess  of  chlorine,  is  used 
in  the  final  test  for  iron. 

The  separation  of  iron,  aluminum,  and  chromium  from  zinc  and 
manganese  is  based  upon  the  fact  that  from  cold  neutral  solu- 
tions of  the  chlorides  of  the  ammonium  sulphide  group,  barium 
carbonate  precipitates  only  the  trivalent  elements.  Ammonium 
carbonate  is  first  added  to  neutralize  the  greater  part  of  the  acid 
and  thus  save  barium  carbonate,  the  subsequent  addition  of 
which  completes  the  neutralization. 


ANALYSIS  69 

In  the  hydrogen  peroxide  tests  for  chromium  the  oxidation  of 
a  chromium  salt  in  alkaline  solution  to  a  chromate  presents  no 
difficulty.  The  subsequent  oxidation  in  acid  solution  to  per- 
chromic  acid  (?)  requires  precise  manipulation.  The  solution 
should  contain  very  little  free  acid  and  the  initial  color  change 
should  be  carefully  observed,  since  the  blue  compound  is  easily 
reduced  by  hydrogen  peroxide  in  the  presence  of  free  acid. 

The  separation  of  aluminum  from  chromium  is  based  upon 
the  difference  in  behavior  of  solutions  of  Cr(ONa)3  and  of 
Al(ONa)3  when  boiled.  The  former  is  decomposed  with  the 
precipitation,  of  Cr(OH)3,  while  Al(ONa)3  is  unchanged  and 
remains  in  solution.  The  sodium  hydroxide  used  in  this  opera- 
tion is  freshly  prepared  from  sodmm  carbonate  and  barium 
hydroxide,  because  the  solvent  action  of  sodium  hydroxide  on 
glass  often  introduces  into  a  solution  of  that  reagent  com- 
pounds of  aluminum  and  silicon  which  would  interfere  with 
the  final  test  for  aluminum. 

The  separation  of  manganese  from  zinc  is  based  upon  the 
solubility  of  manganese  sulphide  in  acetic  acid  and  the  relative 
insolubility  of  zinc  sulphide  in  that  reagent. 

The  9gpk^ioii  of  a  manganous  salt  to  permanganic  acid  by 
lead  dioxide  1%  the  presence  of  sulphuric  acid  is  a  very  delicate 
test.  It  may  be  performed  with  the  original  substance l  if  cer- 
tain precautions  are  observed.  The  presence  of  reducing  agents 
interferes  with  the  reaction.  Hydrochloric  acid  is  capable  of 
reducing  permanganic  acid;  and  since  this  acid  is  set  free 
from  its  salts  by  the  action  of  sulphuric  acid,  the  solution  to  be 
tested  (manganous  chloride  in  the  systematic  course  of  analysis) 
is  boiled  with  concentrated  sulphuric  acid.  Here,  as  in  the 
hydrogen  sulphide  group,  Division  A,  when  decomposition  of 
the  sulphuric  acid  into  sulphur  trioxide  and  water  takes  place,  it 
is  assumed  that  a  more  volatile  acid  has  been  entirely  expelled. 

1  By  original  substance  is  meant  the  substance  under  examination  before  it 
has  been  subjected  to  chemical  treatment, 


70  AMMONIUM   SULPHIDE   GROUP 

Since  manganous  salts  may  be  oxidized,  it  follows  that  they 
are  themselves  reducing  agents ;  they  do,  in  fact,  in  acid  solution 
reduce  permanganic  acid.  It  is  therefore  necessary  to  use  in 
this  test  a  portion  of  the  manganous  solution  so  small  that  the 
amount  of  lead  dioxide  taken  will  suffice  to  oxidize  all  of  the 
manganese  to  permanganic  acid. 

^^—  — 

Analysis  of  the  Ammonium  Sulphide  Group  -when  Phosphates  are  present 

The  phosphates  of  the  ammonium  carbonate  group  and  mag- 
nesium are  soluble  in  dilute  acids  but  are  precipitated  when 
such  solutions  are  made  neutral  or  alkaline.  They  are  insoluble 
in  water. 

It  is  evident,  therefoi^^hat  they  can  be  present  only 
when  an  acid  solution  is  to  be  analyzed  or  when  the  original 
substance  was  insoluble  in  water  but  soluble  in  an  acid.1 

In  the  systematic  precipitation  of  the  various  groups  it  is 
not  until  the  ammonium  sulphide  group  is  reached  that  the 
solution  is  made  alkaline.  The  members  of  the  ammonium 
carbonate  group,  if  present,  will  therefore  be  precipitated  as 
phosphates  in  the  ammonium  sulphide  group,  and  provision 
n^ust  there  be  made  for  their  detection. 

No  modification  of  the  usual  course  of  analysis  is  necessary 
until  after  the  test  for  iron.  The  next  step  in  the  ordinary 
procedure  is  the  addition  of  barium  carbonate  to  the  slightly 
acid  solution.  As  this  always  introduces  barium  chloride 
into  the  filtrate,  it  is  evident  that  a  test  for  barium  must  be 
made  before  the  addition  ^£  barium  carbonate.  After  the 
presence  or  absence  of  barium  has  been  determined,  it  is 
next  necessary  to  separate  phosphoric  acid  from  manganese, 
zinc,  and  the  members  of  the  ammonium  carbonate  group. 
This  is  accomplished  by  taking  advantage  of  the  facts :  (1)  that 
phosphoric  acid  exhibits  a  greater  tendency  to  combine  with 
ferric  iron  than  with  the  above-mentioned  elements;  and  (2)  that 
1  This  is  strictly  true  only  in  the  absence  of  ammonium  salts. 


BAKIUM  71 

when  a  solution  containing  ferric  chloride  and  phosphoric  acid 
is  made  neutral,  ferric  phosphate  is  precipitated.  A  quantity 
of  ferric  chloride  equivalent  to  the  amount  of  phosphoric 
acid  present  is  therefore  added  to  the  solution,  and  the  usual 
course  of  analysis  proceeded  with.  When  the  solution  is 
made  neutral  with  barium  carbonate  all  of  the  phosphoric 
acid  is  then  precipitated  with  the  hydroxides  of  chromium 
and  aluminum  as  ferric  phosphate,  and  the  members  of  the 
ammonium  carbonate  group  pass  into  the  filtrate  and  are 
there  tested  for  in  the  usual  way,  after  the  removal  of  zinc 
and  manganese  if  the  latter  are  present. 


AMMONIUM  CARBONATE  GROUP  AND  MAGNESIUM 

(The  AlkalinejEarths) 
PRELIMINARY  REACTIONS 

BARIUM  :   Reactions  of  solutions  of  barium  salts. 

KOH  or  NaOH  precipitates  from  concentrated  solutions  Ba(OH)2, 
white,  voluminous ;  insoluble  in  excess  of  the  reagent,  but  soluble 
in  water  and  therefore  not  precipitated  from  dilute  solutions. 

NH4OH  produces  no  precipitate. 

Na2C03  or  (NH4)2C03  precipitates  from  neutral  or  alkaline  solutions 
BaC03,  white,  flocculent  at  first.  This  precipitate  becomes  crystalline 
when  gently  wanned,  the  change  taking  place  more  quickly  in  dilute  solu- 
*  tions  than  in  concentrated  ones.  BaC03  is  very  slightly  soluble  in  NH4C1 ; 
soluble  in  HC1  or  acetic  acid,  HC2H302,  with  effervescence.  Barium 
carbonate  is  insoluble  in  excess  of  Na2CO3,  but  is  somewhat 
soluble  in  water  containing  CO2  or  in  a  solution  of  a  bicar- 
bonate, and  therefore  is  not  completely  precipitated  from  acid 
solutions. 

H2S  produces  no  precipitate. 

(NH4)2S  produces  no  precipitate. 

HC1  produces  no  precipitate. 


72  AMMONIUM   CARBONATE   GROUP 

H2S04  or  a  solution  of  a  sulphate,  as  K2S04,  precipitates  BaS04,  white, 
finely  divided l ;  almost  insoluble  in  water  (i  part  in  800,000  parts  of  water) , 
acids,  or  alkalies. 

K2Cr04  precipitates  from  neutral  or  feebly  acid  solutions  BaCr04,  yel- 
low; almost  insoluble  in  water,  slightly  soluble  in  acetic  acid,  soluble  in 
HC1. 

Na2HP04  or  NaNH4HP04  precipitates  from  neutral  or  alkaline  solutions 
Ba3(P04)2  or  BaHP04,  flocculent ;  easily  soluble  in  dilute  HC1  or  HN03. 

(NH4)2C2O4,  ammonium  oxalate,  precipitates  from  concen- 
trated solutions  BaC2O4,  white ;  soluble  in  HC1,  HNO3,  or  hot 
acetic  acid. 

STRONTIUM:  Reactions  of  solutions  of  strontium  salts. 

KOH  or  NaOH  precipitates  from  concentrated  solutions  Sr(OH)2, 
resembling  Ba(OH)2,  but  less  soluble  in  water. 

NH4OH  produces  no  precipitate. 

Na2C03  or  (NH4)2C03  precipitates  SrC03,  resembling  BaC03. 

H^S  produces  no  precipitate. 

(NH4)2S  produces  no  precipitate. 

HC1  produces  no  precipitate. 

H2S04  precipitates  SrS04.  This  substance  resembles  BaS04  but  is  more 
soluble  in  water  (i  part  in  about  7000  parts  of  water).  SrS04  is  more 
soluble  in  HC1  than  it  is  in  water ;  therefore  precipitation  by  H2S04  is  not 
complete,  and  is  less  complete  from  a  solution  containing  hydrochloric  acid 
than  from  a  neutral  solution.  Calcium  sulphate,  although  but  slightly 
soluble  in  water,  is  more  soluble  than  strontium  sulphate,  and  for  this 
reason  a  solution  of  calcium  sulphate  will  precipitate  SrS04  from  a  concen- 
trated solution  of  a  strontium  salt.  Precipitation  is  more  complete  when^ 
the  mixture  is  warmed  or  when  a  concentrated  solution  of  potassium  sul- 
phate is  used  instead  of  the  calcium  sulphate. 

K2Cr04  does  not  precipitate  SrCr04  from  dilute  solutions  acidified  with 
acetic  acid.  SrCr04  is  more  soluble  than  BaCr04  not  only  in  acetic  acid  but 
also  in  water. 

Na2HP04  or  NaNH4HP04  precipitates  phosphates  resembling  those  of 
barium  in  constitution  and  properties. 

(NH4)2C2O4  precipitates  SrC2O4,  white;   soluble  in  HC1  or 
HNO3,  difficultly  soluble  in  acetic  acid. 
-1  See  Introduction,  page  15. 


CALCIUM  73 

CALCIUM  :  Reactions  of  solutions  of  calcium  salts. 

KOH  or  NaOH  precipitates  from  sufficiently  concentrated  cal- 
cium solutions  Ca(OH)2,  similar  to  Sr(OH)2,  but  less  soluble  in 
water. 

NH4OH  produces  no  precipitate. 

N^COgOr  (NH4)2C03  precipitates  CaC03  which  resembles  BaC03.  Cal- 
cium carbonate  is  slightly  soluble  in  an  excess  of  a  concentrated 
solution  of  sodium  carbonate. 

H2S  produces  no  precipitate. 

(NH4)2S  produces  no  precipitate. 

HC1  produces  no  precipitate. 

H2S04  or  a  soluble  sulphate  precipitates  CaS04  only  from  concentrated 
solutions,  and  then  but  partially,  on  account  of  its  solubility  in  water  (i 
part  in  about  500  parts  of  water). 

K2Cr04  produces  no  precipitate  in  dilute  solutions  acidified  with  acetic 
acid,  CaCr04  being  readily  soluble  in  water  and  also  in  acetic  acid. 

Na2HP04  or  NaNH4HP04  precipitates  phosphates  resembling  those  of 
barium  and  strontium. 

(NH4)2C204  precipitates  CaC204,  white,  crystalline  ;  insoluble  in  water, ] 
soluble  in  HC1  or  HN03.  Calcium  oxalate  is  somewhat  less  soluble 
in  acetic  acid  than  are  the  oxalates  of  barium  and  strontium. 

MAGNESIUM:  Reactions  of  solutions  of  magnesium  salts. 

KOH  or  NaOH  precipitates  Mg(OH)2,  white ;  almost  insoluble 
in  water. 

NH4OH  precipitates  from  neutral  solutions  containing  no  ammonium 
salts  one-half  of  the  magnesium  as  Mg(OH)2.  The  other  half  unites 
with  the  ammonium  salt  that  is  formed,  producing  double  salts  such  as 
MgCl2-2NH4Cl.  These  double  salts  are  soluble  in  water  and  are  unaffected 
by  NH4OH.  In  analytical  practice  there  is  usually  present  sufficient 
NH4C1  to  unite  with  all  of  the  magnesium,  forming  MgCl2-2NH4Cl,  and 
therefore  in  the  actual  analysis  ammonium  hydroxide  does  not  precipitate 
magnesium. 

Na2C03  or  (NH4)2C03,  in  the  absence  of  other  ammonium  salts, 
precipitates  white  basic  carbonates ;  soluble  in  large  excess  of  the 


74  AMMONIUM  CARBONATE  GROUP 

concentrated  reagents.  From  solutions  containing  magnesium  ammo- 
nium double  sa/ts,  magnesium  is  not  precipitated  by  ammonium 
carbonate. 

H2S  produces  no  precipitate. 

(NH4)2S  produces  no  precipitate. 

HC1  produces  no  precipitate. 

H2S04  produces  no  precipitate. 

Na2HP04  or  NaNH4HP04  precipitates  white  flocculent  phosphates 
similar  to  those  of  barium.  From  solutions  of  double  salts,  such  as 
MgCl^  NH4C1,  in  the  presence  of  NH4OH,  an  alkali  phosphate  pre- 
cipitates MgNH4P04,  white,  crystalline;  somewhat  soluble  in  water, 
easily  soluble  in  acids,  insoluble  in  NH4OH.  The  crystalline  condition 
of  the  precipitate  is  most  apparent  when  precipitation  takes  place  slowly 
from  dilute  solutions.  Crystallization  is  hastened  by  stirring  the  solu- 
tion with  a  glass  rod.  If  the  solution  is  very  dilute  the  precipitate  will 
first  appear  where  the  rod  has  touched  the  inner  surface  of  the  test  tube. 


ANALYSIS 


75 


METHOD  or  ANALYSIS 

The  filtrate  from  the  ammonium  sulphide  group  may  be 
colored  by  ammonium  sulphide,  see  page  76,  or  by  some  of  the 
members  of  the  previous  group  soluble  in  that  reagent  or  in 
NH4OH.  If  so,  it  should  be  boiled  until  the  odor  of  H2S  has 
disappeared  and  the  solution  is  colorless,  then  concentrated  to 
small  bulk  and  filtered.  This  solution,  or  the  original  solution 
in  case  it  is  known  that  only  members  of  this  group  are  present, 
is  then  treated  as  follows : 

To  the  solution  add  NH4C1,  NH4OH,  and  then  (NH4)2CO8.  Warm  the  solu- 
tion.  If  a  precipitate  appears,  warm  gently  until  it  becomes  crystalline.  (If  no 
precipitate  appears,  test  the  solution  for  magnesium  and  the  group  of  the  alkali 
metals  unless  these  are  known  to  be  absent. )  If  a  precipitate  appears,  filter. 


Precipitate  :  Barium,  Strontium,  Calcium 

Filtrate:   Magne- 

Place the  precipitate  in  a  beaker  and  dissolve  it  in  warm 

sium  (and  Alkali 

acetic  acid,  avoiding  a  large  excess  of  the  acid.     To  a  small 

Metals) 

portion  of  the  solution  add  K2Cr04.     (If  no  precipitate 

Divide  into  three 

appears,  test  the  remainder  of  the  solution  for  strontium 

portions.      Reserve 

and  calcium.)     If  a  precipitate  is  formed,  add  K2CrO4  to 

the    larger    portion 

the  remainder  of  the  solution,  warm  gently,  and  filter. 

for  the  detection  of 

Precipitate  : 

Filtrate  :  Strontium,  Calcium 

the    alkali    metals, 
unless    these    are 

Barium 

Make    alkaline    with    NH4OH,   add 

known  to  be  absent. 

Dissolve    in 
HC1,    warm  the 

(NH4)2CO3,  and  warm.     (If  no  precipi- 
tate appears,  calcium  and  strontium  are 

To  a  small  portion 
add  NH4OH  and  a 

solution,   and 

absent.)     If  a  precipitate  appears,  filter, 

little    NaNH4HP04. 

add  a  few  drops 

wash  thoroughly,  and  dissolve  on  the 

Rub  the  inner  sur- 

of   H2S04.       A 

filter  with  the  smallest  possible  quantity 

face  of  the  test  tube 

white   precipi- 
tate   proves   the 
presence    of    ba- 
rium. 

of  HC1.     Evaporate  just  to  dryness,  dis- 
solve in  a  little  water,  filter  if  not  clear, 
and    evaporate    to    small    bulk    (about 
2  cc.).      Divide  into  two  portions. 

with  a  glass  rod.    A 
crystalline    precipi- 
tate proves  the  pres- 
ence of  magnesium. 

(The    precipi- 

Portion 1 

Portion  2 

If  the  precipitate  is 

tate  will  appear 
yellow  from  the 

Strontium 

Calcium 

flocculent,  add  to  an- 
other portion  dilute 

K2CrO4   in  the 

Add    a    small 

Add  K2S04  and 

H2SO4    until    acid, 

solution.) 

amount  of  a  solu- 

heat to  boiling.     If 

and  filter  if  a  pre- 

tion of  CaS04,  heat 

a  precipitate  is 

cipitate   remains; 

to  boiling,  and  if  no' 

formed,    filter,    re- 

make the  filtrate  al- 

precipitate appears 

ject  the  precipitate, 

kaline  with  NH4OH, 

immediately,  let 

and  to  the  filtrate 

add      (NH4)2C2O4, 

stand  for  ten  min- 

add    NH4OH     till 

warm,  filter  if  a  pre- 

utes.    A  fine  white 
precipitate    proves 

alkaline,      then 
(NH4)2C2O4,     and 

cipitate  appears,  and 
test  the  filtrate  for 

the  presence  of 

warm.     A  white 

magnesium    with 

strontium. 

crystalline  precipi- 

NaNH4HP04. 

tate  proves    the 

presence  of  calcium. 

76  AMMONIUM  CARBONATE  GROUP 

DISCUSSION 

[The  filtrate  from  the  preceding  group  contains  ammonium 
sulphide.  If  this  reagent  has  been  freshly  prepared,  its  presence 
will  not  interfere  with  the  further  analysis  of  the  solution. 
But  ammonium  sulphide  decomposes  on  long  standing  and 
there  is  formed  a  yellow  polysulphide.  When  such  a  yellow 
ammonium  sulphide  has  been  used  in  precipitating  the  pre- 
ceding group  and  the  filtrate  from  that  group  is  boiled,  sulphur 
is  deposited  and  may  interfere  with  the  subsequent  detection  of 
the  ammonium  carbonate  group.  Therefore,  if  the  filtrate  from 
the  ammonium  sulphide  group  is  yellow  in  color  it  is  desirable, 
before  proceeding  with  the  analysis,  to  decompose  the  ammonium 
polysulphide  by  boiling.  The  precipitated  sulphur  should  then 
be  removed  by  filtration.  Moreover,  the  filtrate  from  the 
ammonium  sulphide  group  may  contain  nickel  or  chromium, 
the  presence  of  nickel  being  due  to  the  solubility  of  nickel 
sulphide  in  ammonium  sulphide  with  the  formation  of  a  brown 
solution,  while  that  of  chromium  is  due  to  the  solvent  action  of 
ammonium  hydroxide  and  ammonium  chloride  upon  chromium 
hydroxide,  a  red  solution  containing  CrCl3-4  NH3  being  formed. 
The  filtrate  from  the  ammonium  sulphide  group  may  also  con- 
tain aluminum  because  of  the  solubility  of  aluminum  hydroxide 
in  ammonium  hydroxide.  All  of  these  solvents  are  volatile, 
and  therefore  when  the  solution  is  boiled  down  to  small  bulk 
they  are  removed,  and  any  compound  of  nickel,  chromium,  or 
aluminum  that  may  have  been  present  in  the  filtrate  from  the 
ammonium  sulphide  group  will  be  precipitated.] 

The  members  of  the  ammonium  carbonate  group,  magnesium, 
and  the  alkali  metals  are  not  precipitated  from  solutions  of 
their  salts  by  hydrochloric  acid,  hydrogen  sulphide,  or  ammonium 
sulphide.1 

1  The  precipitation  of  the  members  of  the  ammonium  carbonate  group  as 
oxalates,  phosphates,  etc.,  in  the  ammonium  sulphide  group  has  already  been 
discussed  (pages  66  and  70). 


ANALYSIS  77 

The  insolubility  in  water  of  the  carbonates  of  the  ammonium 
carbonate  group  is  made  the  basis  of  the  separation  of  these 
elements  from  the  alkali  metals. 

The  separation  of  barium  from  strontium  and  calcium  is 
based  on  the  fact  that  barium  chromate  is  but  slightly  soluble 
in  acetic  acid,  while  the  chromates  of  strontium  and  calcium 
are  easily  soluble  in  that  reagent.  A  large  excess  of  acid  will, 
however,  dissolve  sufficient  barium  chromate  to  interfere  with 
the  later  test  for  strontium  and  should  be  avoided. 

The  nitrate  from  the  barium  chromate  precipitate  contains 
acetic  acid,  potassium  acetate,  and  potassium  chromate,  as  well 
as  the  acetates  of  strontium  and  calcium  if  present.  In  order 
to  obtain  a  solution  containing  only  strontium  and  calcium, 
these  elements  are  reprecipitated  as  carbonates,  which  are  then 
changed  to  chlorides  by  treatment  with  hydrochloric  acid. 

The  fact  that  calcium  sulphate  is  more  soluble  than  strontium 
sulphate  is  made  use  of  in  the  separation  of  calcium  from 
strontium  and  in  the  test  for  strontium.  Since  both  of  these 
sulphates  are  somewhat  soluble  in  water  and  still  more  so  in 
hydrochloric  acid,  it  is  desirable  that  the  solution  should  be 
concentrated  and  neutral.  This  condition  is  attained  by  evap- 
orating the  solution  to  dryness  and  redissolving  in  but  little 
water.  From  this  solution  of  the  chlorides,  calcium  sulphate 
will  precipitate  strontium  sulphate. 

In  order  to  remove  the  strontium  from  the  portion  of  the 
solution  of  the  chlorides  which  is  to  be  tested  for  calcium,  a 
strong  solution  of  potassium  sulphate  is  used.  This  precipitates 
the  strontium  almost  completely  and  may  also  precipitate  some 
of  the  calcium,  but  enough  of  the  latter  to  give  the  calcium  test 
will  always  pass  into  the  nitrate  because  of  the  greater  solubility 
of  calcium  sulphate. 

A  comparison  of  the  solubilities  of  those  salts  of  barium, 
strontium,  and  calcium  which  are  involved  in  their  separation 
shows  that  the  test  for  strontium  assumes  the  previous  removal 


78  AMMONIUM  CARBONATE   GROUP 

of  barium,  and  that  the  test  for  calcium  assumes  the  previous 
removal  of  strontium.  As  has  been  seen,  the  separation  of  these 
elements  by  the  above  method  is  not  altogether  sharp  and  could 
not  be  used  to  detect  traces  of  the  members  of  this  group.  If 
used  with  care,  however,  it  is  sufficiently  accurate  for  all  ordi- 
nary purposes. 

Magnesium  is  distinguished  from  barium,  strontium,  and 
calcium  by  the  greater  solubility  of  its  carbonate  in  ammonium 
chloride.  If  sufficient  ammonium  chloride  is  present  to  form 
a  double  salt,  as  MgCl2-  2  NH4C1,  ammonium  carbonate  does  not 
precipitate  magnesium,  and  this  element  passes  with  the  members 
of  the  group  of  the  alkali  metals  into  the  nitrate.  Sufficient 
ammonium  chloride  to  form  the  double  salt  MgCl2-2NH4Cl 
is  usually  present  in  the  nitrate  from  the  preceding  group. 

It  should  be  borne  in  mind  that  the  carbonates  of  barium, 
strontium,  and  calcium  are  slightly  soluble  in  the  presence  of 
a  large  excess  of  ammonium  chloride.  The  addition  of  sodium 
ammonium  phosphate  in  the  test  for  magnesium  will  produce  a 
flocculent  precipitate  in  case  traces  of  barium,  strontium,  or 
calcium  have  passed  through  into  the  nitrate  from  the  ammo- 
nium carbonate  group.  These  elements  may  be  removed  almost 
completely,  with  the  exception  of  traces  of  strontium,  by  taking 
advantage  of  the  insolubility  of  the  sulphates  of  barium  and 
strontium  and  of  the  oxalates  of  strontium  and  calcium.  The 
precipitate  may  also  be  flocculent  (1)  if  any  aluminum  has 
been  dissolved  by  the  use  of  an  excessive  amount  of  ammonium 
hydroxide  in  the  precipitation  of  the  ammonium  sulphide  group 
and  has  not  been  removed  by  the  treatment  prescribed  on  page 
75 —  since  aluminum  phosphate  is  insoluble  in  acetic  acid,  its 
removal  from  magnesium  ammonium  phosphate  is  easy  —  or 
(2)  if  the  quantity  of  magnesium  is  so  great  that  the  addition 
of  the  reagent  produces  an  immediate  precipitation,  in  which 
case  the  precipitate  may  be  dissolved  in  an  acid,  the  solution 
diluted,  and  the  test  for  magnesium  repeated. 


AMMONIA  79 

AMMONIA,    SODIUM,    POTASSIUM 

(The  Alkalies) 

PRELIMINARY   REACTIONS 

AMMONIA:    Reactions  of  solutions  of  ammonium  salts. 

No  precipitate  is  produced  by  NH4OH,  Na^Og,  H^S,  (NH4)2S, 
HC1,  H^. 

KQH  or  NaOH  when  warmed  with  an  ammonium  salt  in  the  solid  form 
or  in  solution  liberates  ammonia  gas.  Moist,  red  litmus  paper  turns  blue, 
and  moist  turmeric  paper  turns  brown,  when  exposed  to  NH3. 

Co(N02)3'3NaN02,  sodium  cobaltic  nitrite,  precipitates  from  acetic 
acid  solutions  Co(N02)3'3NH4N02  in  the  form  of  a  yellow  powder. 

When  an  ammonium  salt  is  heated  in  a  porcelain  dish  over 
the  Bunsen  flame  it  is  decomposed  into  volatile  constituents. 
The  chloride  is  dissociated  into  NH3  and  HC1,  which  are  volatile 
and  reunite  in  the  cool  air  above  the  dish,  forming  a  white  cloud. 
Other  ammonium  salts  when  heated  lose  their  nitrogen  either 
as  free  nitrogen,  ammonia,  or  an  oxide  of  nitrogen. 

SODIUM:   Reactions  of  solutions  of  sodium  salts. 

No  precipitate  is  produced  by  KOH,  NaOH,  NH4OH,  Na2C03,  H2S, 
(NH4)2S,  HC1,  H2S04. 

Co(N02)3'3NaN02  produces  no  precipitate  in  acetic  acid  solutions  of 
sodium  salts. 

Flame  test.  If  a  sodium  salt  is  brought  upon  a  platinum  wire  and 
introduced  into  the  Bunsen  flame,  the  flame  is  colored  a  brilliant  yellow. 

Sodium  salts  are  but  slightly  volatilized  when  heated  in  a 
porcelain  dish  over  the  Bunsen  flame. 

POTASSIUM:    Reactions  of  solutions  of  potassium  salts. 

No  precipitate   is  produced  by  KOH,  NaOH,  NH4OH,  Na2C03, 
,  (NH4)2S,  HC1,  H2S04. 


80  THE  ALKALIES 

Co(N02)3-3  NaN02  when  added  to  a  concentrated  solution  of  a  potassium 
salt  containing  free  acetic  acid  but  no  free  inorganic  acid,  precipitates  at 
once  Co(N02)3'3KN02  in  the  form  of  a  yellow  powder.  Precipitation 
takes  place  slowly  from  dilute  solutions,  but  is  hastened  by  gently  warm- 
ing the  mixture.  If  the  solution  is  alkaline,  it  should  be  acidified  with 
acetic  acid  before  the  addition  of  the  sodium  cobaltic  nitrite  :  if  it  is  acid, 
it  should  first  be  made  neutral  by  the  addition  of  sodium  carbonate  and 
should  then  be  acidified  with  acetic  acid. 

Flame  test.  If  a  potassium  salt  is  brought  upon  a  platinum  wire  and 
introduced  into  the  Bunsen  flame,  the  flame  is  colored  violet.  A  very 
small  quantity  of  sodium  completely  masks  the  color  ;  but  if  the  flame  is 
observed  through  a  sufficiently  thick  layer  of  blue  glass,  the  yellow  sodium 
rays  are  absorbed,  while  the  potassium  rays  pass  through  and  may  be 
recognized. 

Potassium  salts  are  but  slightly  volatilized  when  heated  in  a 
porcelain  dish  over  the  Bunsen  flame. 


DETECTION 

m 

1  «&' 

Ammonia.  Place  a  small  portion  of  the  original  substance,1 
whether  it  be  in  the  solid  form  or  in  solution,  in  a  small  beaker 
and  add  NaOH.  Cover  the  beaker  with  a  watch  glass  on  the 
underside  of  which  is  placed  a  moistened  piece  of  turmeric 
paper  or  of  red  litmus  paper.  (If,  in  making  this  test,  the 
student  uses  a  paper  which  he  himself  has  prepared  by  treating 
blue  litmus  paper  with  an  acid,  the  paper  should  be  thoroughly 
washed  until  it  is  free  from  acid.)  Heat  gently  until  the  first 
bubble  appears  in  the  liquid,  and  then  remove  the  beaker  from 
the  flame.  If  the  turmeric  paper  is  turned  brown  or  the  litmus 
paper  blue,  the  presence  of  ammonia  is  proved. 

Sodium  and  potassium.  Evaporate  the  filtrate  from  the  ammo- 
nium carbonate  group  to  dryness,  and  heat  the  solid  residue  in 
a  porcelain  dish  directly  over  a  Bunsen  flame  until  white  fumes 
are  no  longer  given  off.  With  a  glass  rod  scrape  into  the 

1  By  original  substance  is  meant  the  substance  under  examination  before  it 
has  been  subjected  to  chemical  treatment. 


ANALYSIS  81 

bottom  of  the  evaporator  any  of  the  substance  that  may  adhere 
to  the  sides  of  the  dish,  and  heat  again.  Continue  this  heat- 
ing  for  ten  minutes  after  white  fumes  have  ceased  to  appear. 
Divide  into  two  portions  the  dry  residue  thus  obtained  and 
test  this  substance  for  sodium  and  potassium  in  the  manner 
described  below  (Portions  1  and  2). 

If  the  original  substance  is  known  to  contain  members  of 
this  group  only  and  ammonia  has  been  ,shown  to  be  absent,  the 
substance  may  be  tested  as  directed  below  without  previous 
heating. 

Portion  i :  Sodium  (Potassium).  Moisten  a  small  portion  of  the 
residue  with  HC1,  bring  it  upon  a  platinum  wire,  and  introduce 
it  into  the  Bunsen  flame.  A  bright  yellow  flame  proves  the 
presence  of  sodium.  A  violet  flame  proves  the  presence  of 
potassium. 

If  sodium  is  present,  observe  the  flame  through  a  layer  of 
blue  glass  thick  enough  to  absorb  the  sodium  rays.  A  violet 
flame  proves  the  presence  of  potassium. 

Portion  2  :  Potassium.  Dissolve  the  remainder  of  the  residu .5 
in  very  little  water.  If  the  solution  is  alkaline,  make  slight  iy 
acid  with  acetic  acid.  Add  a  few  drops  of  sodium  cobaltic 
nitrite,  warm  very  gently,  and  allow  to  stand  for  a  few  minutes. 
A  yellow  precipitate  proves  the  presence  of  potassium. 

DISCUSSION 

When  testing  the  original  substance  with  sodium  hydroxide 
for  ammonia,  care  should  be  taken  that  none  of  the  sodium 
hydroxide  comes  in  contact  with  either  the  turmeric  paper  or 
the  red  litmus  paper. 

The  flame  test  for  sodium  is  so  delicate  and  that  element  is 
so  widely  distributed  that  care  must  be  exercised  to  distinguish 
the  brilliant  and  permanent  coloration  due  to  a  sodium  com- 
pound in  the  mixture  under  analysis  from  the  slight  yellow 


82  THE  ALKALIES 

coloring  of  the  flame  due  to  the  sodium  chloride  which  is 
always  present  in  the  air  of  the  room  and  in  dust. 

If  a  large  amount  of  sodium  is  present,  it  is  not  always  easy 
to  draw  accurate  conclusions  from  the  flame  test  for  potassium 
unless  the  blue  glass  is  of  exactly  the  proper  shade.  Unless 
such  glass  is  used,  more  weight  should  be  given  to  the  test  with 
sodium  cobaltic  nitrite. 

The  filtrate  from  the  ammonium  carbonate  group  always 
contains  ammonium  salts  because  these  have  been  introduced 
as  reagents  during  the  course  of  the  analysis.  These  ammo- 
nium compounds  will  form  with  sodium  cobaltic  nitrite  a  pre- 
cipitate similar  in  appearance  to  that  produced  by  potassium 
salts,  and  it  is  therefore  necessary  to  remove  them  completely 
before  testing  for  potassium. 


PART   III 

THE  ACIDS 


(Use  solutions  of  the  alkali   metals   in   performing  the  preliminary 
reactions.     In  the  directions  for  the  detection  of  the  acids  reference  will  be 
made  to  solutions  A,  B,  etc.     The  preparation  of  solutions  thus  designated 
is  discussed  on  page  117.) 
41 

CHLORATES 

All  chlorates  are  soluble  in  water.  If  a  chlorate  is  introduced 
into  a  concentrated  solution  of  sodium  carbonate  and  that  solu- 
tion is  then  boiled,  transposition  takes  place  and  sodium  chlorate 
is  formed. 

When  chlorates  are  highly  heated  they  are  decomposed, 
oxygen  being  set  free  and  chlorides  being  formed. 

If  a  solution  of  a  chlorate  is  acidified  and  is  then  colored 
blue  by  the  addition  of  a  drop  of  a  solution  of  indigo,  the 
subsequent  addition  of  Na2SO3  destroys  the  color. 

If  a  small  crystal  of  a  chlorate  is  placed  in  a  dry  test  tube 
and  a  few  drops  of  concentrated  sulphuric  acid  are  then  added, 
a  complicated  reaction  results  and  there  is  produced  a  greenish 
yellow  gas,  C1O2.  This  gas  is  very  unstable  and  it  explodes 
with  considerable  violence  when  the  test  tube  is  warmed  over 
the  Bunsen  flame.  This  experiment  should  be  carried  on  under 
the  hood  and  the  mouth  of  the  tube  should  be  directed  away 
from  the  operator. 

DETECTION 

Acidify  a  small  portion  of  solution  A  with  H2SO4  and  test 
with  indigo  solution  and  Na2SO3.  If  the  blue  color  is 

83 


84 


THE  ACIDS 


permanent,  chlorates  are  absent.  If  the  solution  is  bleached, 
they  may  be  present,  and  a  small  portion  of  the  original  solid 
substance  should  be  tested  with  concentrated  H2SO4  in  the 
manner  above  described. 

DISCUSSION 

A  number  of  substances  other  than  chlorates  will  decolorize 
an  indigo  solution. 

The  treatment  of  a  solid  substance  with  concentrated  sul- 
phuric acid  may  give  rise  to  reactions  that  furnish  valuable 
information  concerning  the  possible  presence  of  substances 
other  than  chlorates.  Some  of  these  reactions  are  tabulated 
below. 


Product  of 
Reaction 


Due  to 


Recognized 
by 


C02 

CO 

HCN 

S02 

S02 

H2S 


C102 

a        \ 

Br 

I 

N205 

HC1 

HC2H3O2 


carbonate  or  oxalate 
oxalate  or  ferrocyanide 
cyanide,  ferrocyanide,  or  ferri- 

cyanide 
sulphite,     or     strong    reducing 

agent 
tartrate 


sulphide,  or  sulphite  and  strong 

reducing  agent 
sulphide  and  oxidizing  agent 
sulphite  and  reducing  agent 
chlorate 

chloride  and  oxidizing  agent 
chlorate  and  reducing  agent 
bromide 
iodide 
nitrate 
chloride 
acetate 


lime  water 

blue  color  of  flame 

odor.     Caution:    This  gas  is 

very  poisonous. 
odor 

odor  of  burnt  sugar  as  well 
as  of  S02;  liquid  black- 
ened 

odor 

burns  to  S02 
color;  explosion 
color;  odor 

color ;  condenses  to  liquid 

color ;  condenses  to  solid 

color 

odor 

odor 


CARBONATES  85 


CARBONATES 

All  normal  carbonates  except  those  of  the  alkalies  are 
insoluble  in  water. 

The  carbonates  of  arsenic,  antimony,  tin,  chromium,  and 
aluminum  are  rare  or  unknown.  K2CO3  is  deliquescent. 
(NH4)2CO3  is  volatile. 

Most  carbonates  are  easily  decomposed  by  hydrochloric  acid  with  the 
evolution  of  C02. 

If  a  drop  of  lime  water  in  a  loop  tube 1  is  exposed  to  C02,  it  becomes 
turbid  through  the  formation  of  CaC03. 

DETECTION 

If  carbonates  have  not  already  been  detected  in  the  analysis  for 
the  bases  or  in  the  preparation  of  solutions  for  the  same,  place 
a  small  portion  of  the  original  solid  substance  in  a  test  tube,  add 
a  little  HC1,  and  hold  hi  the  upper  part  of  the  test  tube  a  loop 
tube  carrying  a  film  of  lime  water.  If  the  presence  of  carbon 
dioxide  is  not  indicated,  warm  the  contents  of  the  test  tube 
and,  if  the  drop  of  lime  water  still  remains  clear,  add  a  little 
HCI  and  again  gently  warm  the  liquid. 

DISCUSSION 

CO2  in  excess  dissolves  the  precipitate  that  is  at  first 
produced  in  the  film,  probably  forming  an  acid  carbonate, 
CaH2(CO3)2.  The  film  should  therefore  be  examined  shortly 
after  its  exposure  to  the  gas. 

1  The  loop  tube  is  made  from  a  piece  of  glass  tubing  4  mm.  in  diameter  by 
heating  the  tube  about  an  inch  from  one  end  in  the  flame  of  the  blast  lamp, 
drawing  it  out  quickly,  and  turning  it  back  upon  itself  in  the  form  of  a  shep- 
herd's crook,  making  a  loop  about  3  mm.  in  diameter.  The  tube  is  then  cut  off 
at  the  end  of  this  crook.  To  fill  the  loop,  hold  the  tube  upright  and  pour  upon 
the  small  encLthe  reagent  that  is  to  be  used'.  This  should  be  done  in  such  a 
way  as  to  force  some  of  the  liquid  to  rise  in  the  main  portion  of  the  tube  and 
to  cause  a  film  to  form  across  the  loop. 


86  THE  ACIDS 

Certain  carbonates  are  not  very  readily  attacked  by  cold 
dilute  HC1.  Among  these  are  the  carbonates  of  silver,  lead, 
mercury  (mercurous),  copper,  bismuth,  and  iron.  These  car- 
bonates are,  however,  attacked  by  warm  dilute  hydrochloric  acid 
or  by  warm  HCI. 

,  Care  must  be  taken  as  to  the  position  of  the  loop  in  the 
test  tube,  especially  if  the  tube  is  warmed  or  HCI  is  used, 
since  hydrochloric  acid  carried  up  by  effervescence  or  evolved 
as  a  gas  dissolves  the  precipitate  in  the  film  or  prevents  its 
formation. 

A  slight  turbidity  in  the  film  is  best  observed  against  a  dark 
background. 

SULPHIDES 

All  normal  sulphides  are  insoluble  in  water  except  those  of 
the  alkalies. 

By  the  action  of  HCI,  hydrogen  sulphide  is  readily  evolved 
from  the  sulphides  of  the  alkalies,  alkaline  earths,  magnesium, 
zinc,Jmanganese,  and  iron;  less  readily  from  the  sulphides  of 
lead,  msmuth,  cadmium,  antimony,  tin,  nickel,  and  cobalt; 
from  other  sulphides  with  difficulty  or  not  at  all. 

Most  of  the  sulphides  that  are  not  readily  attacked  by  HCI 
alone  yield  H2S  when  treated  with  zinc  and  hydrochloric  acid. 

H2S  blackens  filter  paper  that  has  been  moistened  with  lead  ammonium 
acetate. 

By  prolonged  action  of  hot  HNO%  most  sulphides  are  decom- 
posed, usually  with  the  separation  of  sulphur  and  the  formation 
of  nitrates  and  sulphuric  acid :  PbS  is  partially  oxidized  to  the 
sulphate,  HgS  is  changed  to  Hg(NO3)2-2  HgS,  and  the  sul- 
phides of  arsenic,  antimony,  and  tin  are  changed  to  oxides  or  to 
H3AsO4,  H3SbO4,  and  H2SnO3. 

Aqua  regia  decomposes  the  sulphides  with  the  separation  of 
sulphur  and  the  formation  of  sulphuric  acid. 


SULPHITES  87 

When  an  insoluble  sulphide  is  fused  with  a  small  piece  of  NaOH  on  a 
porcelain  crucible  cover,  Na2S  is  formed.  If  this  residue  is  dissolved  in 
water  and  a  drop  of  the  solution  is  placed  on  a  silver  coin,  a  black  stain 
of  Ag2S  appears. 

DETECTION 

If  sulphides  have  not  already  been  detected  in  the  analysis 
for  the  bases,  or  in  the  preparation  of  solutions  for  the  same,  or 
in  the  tests  for  a  chlorate  or  a  carbonate,  they  may  usually  be 
detected  as  follows :  add  HC1  to  a  small  portion  of  the  original 
solid  substance  in  a  test  tube,  warm  gently,  and  cover  the  mouth 
of  the  tube  with  a  piece  of  filter  paper  that  has  been  moistened 
with  a  drop  of  lead  ammonium  acetate  solution. 

If  the  result  of  the  test  is  negative  and  a  colored  residue 
remains  hi  the  test  tube,  add  a  granule  of  zinc  and  repeat 
the  test. 

DISCUSSION 

A  few  sulphides  are  not  decomposed  by  hydrochloric  acid 
even  after  the  addition  of  zinc.  Therefore,  if  the  results  of 
the  above  tests  for  a  sulphide  were  negative  and  a  colored 
residue  still  remains  in  the  test  tube,  the  residue  should  be 
separated  by  filtration,  washed  thoroughly,  and  dried.  It  is 
then  fused  with  a  small  piece  of  NaOH  on  a  porcelain  crucible 
cover.  The  fused  mass  is  allowed  to  cool,  removed  from  the 
cover,  placed  upon  a  clean  silver  coin,  and  moistened  with  a 
drop  of  water. 


SULPHITES 

The  sulphites  of  the  alkalies  are  deliquescent  and  are  the 
only  sulphites  that  are  easily  soluble  in  water.  All  sulphites  are 
transposed  by  boiling  with  Na2CO3.  By  the  action  of  the  air  or 
of  other  oxidizing  agents  sulphites  are  oxidized  to  sulphates. 


88  THE  ACIDS 

All  sulphites  are  decomposed  by  HC1  with  the  evolution 
of  S02. 

If  a  drop  of  a  mixture  of  FeCl3  and  K8Fe(CN)6  in  a  loop 
tube  is  exposed  to  SO2,  the  FeCl3  is  reduced  to  FeCl2  which 
then,  with  the  K8Fe(CN)6,  forms  Fe3[Fe(CN)6]2  which  is  blue. 

DETECTION 

If  not  already  detected  in  the  analysis  for  the  bases  or  in 
the  preparation  of  solutions  for  the  same,  or  in  the  tests  for  a 
chlorate,  a  carbonate,  or  a  sulphide,  acidify  a  portion  of  solution 
A  with  HC1  and  test  the  evolved  gas  as  above  directed. 

DISCUSSION 

FeCl3  is  also  reduced  by  H2S.  The  evolution  of  this  gas 
may  be  prevented  as  follows :  if  a  sulphide  is  present,  make 
a  portion  of  solution  A  nearly  neutral  by  the  addition  of  HC1, 
then  add  a  little  HgCl2,  and  finally  complete  the  acidification 
and  test  the  evolved  gas  for  SO2. 


CYANIDES 

The  cyanides  of  the  alkalies  and  alkaline  earths,  and  mercuric 
cyanide,  are  soluble  in  water;  most  other  cyanides  are  insoluble. 

All  cyanides  are  transposed  by  boiling  with  Na2CO3. 

From  a  solution  of  a  cyanide  of  an  alkali  metal,  AgNO3 
precipitates  AgCN,  white ;  insoluble  in  dilute  HNO3 ;  soluble 
in  KCN,  forming  AgCN-KCN ;  soluble  in  NH4OH  or  (NH4)2CO3, 
probably  forming  AgCN  •  2  NH3. 

AgCN  is  decomposed  when  heated  to  dull  redness,  metallic 
silver  and  cyanogen  being  formed. 

Hydrocyanic  acid,  HCN,  a  colorless  and  poisonous  gas,  is 
liberated  when  dilute  sulphuric  acid  acts  upon  a  cyanide  of  an 


CYANIDES  89 

alkali.  If  a  drop  of  (NH4)2SX  in  a  loop  tube  is  exposed  to  this 
gas  for  a  few  minutes,  NH4CNS  is  formed.  After  the  excess 
of  (NH4)2SX  has  been  expelled  by  evaporating  the  drop  to 
dryness  on  a  porcelain  crucible  cover,  the  addition  of  a  drop 
of  a  dilute  solution  of  FeCl3  to  the  cooled  residue  produces  the 
deep  red  Fe(CNS)3. 

When  a  solution  of  a  cyanide  of  ag.  alkali  metal  (KCN)  is 
made  strongly  alkaline  with  KOH  or  NaOH,  and  a  little  FeSO4 
and  a  drop  of  FeCl3  are  then  added,  a  precipitate  is  produced 
which  consists  of  ferrous  and  ferric  hydroxides.  If  the  mixture 
is  warmed,  the  .ferrous  hydroxide  is  changed  to  Fe(CN)2  which 
then  combines  with  KCN  to  form  K4Fe(CN)6.  If  now  the 
mixture  is  acidified  with  HC1,  the  hydroxides  of  iron  dissolve 
and  the  deep  blue,  insoluble  ferric  ferrocyanide  appears. 

DETECTION 

If  not  already  detected  in  the  analysis  for  the  bases,  or  in  the 
preparation  of  solutions  for  the  same,  or  in  the  test  for  a  carbo- 
nate, add  dilute  H2SO4  to  a  small  portion  of  solution  A  and 
warm  gently.  Test  the  gas  with  (NH4)2SX,  etc.,  as  described 
above.  The  loop  tube  should  here  be  supported  by  thrusting 
its  stem  into  a  cork,  and  to  relieve  gas  pressure  in  the  tube  the 
cork  should  have  a  shallow  channel  cut  along  its  side.  The 
film  should  be  exposed  to  the  action  of  the  gas  for  ten  minutes. 

If  ferrocyanides  and  ferricyanides  are  absent,  the  ferric  ferro- 
cyanide test  for  a  cyanide  should  also  be  made,  a  small  portion 
of  solution  A  being  used  for  the  purpose. 

DISCUSSION 

In  the  sulphocyanate  test  the  excess  of  (NH4)2SX  must  be 
expelled  by  evaporation  to  prevent  the  formation  of  FeS  when 
FeCl3  is  added. 


90  THE  ACIDS 

Unless  the  crucible  cover  is  cooled  before  the  addition  of  the 
drop  of  FeCl3,  the  latter  will  be  evaporated  to  dryness  by  the 
hot  cover  and  will  leave  a  reddish  residue  that  may  mislead 
the  student. 

PHOSPHATES 

The  phosphates  of  the  alkalies  are  soluble  in  water.  Normal 
phosphates  of  the  other  metals  are  insoluble  in  water,  but  are 
readily  soluble  in  dilute  inorganic  acids. 

Some  phosphates  are  not  readily  transposed  by  boiling  with 
Na2C03. 

From  a  solution  of  an  orthophosphate  to  which  nitric  acid  has 
been  added,  ammonium  molybdate,  (NH4)2Mo04,  precipitates  ammonium 
phospho-molybdate,  lemon  yellow,  pulverulent,  adhering  more  or  less  to 
the  walls  of  the  test  tube.  The  composition  of  the  precipitate  varies 
according  to  the  conditions  under  which  precipitation  takes  place.  Pre- 
cipitation is  best  obtained  by  adding  a  small  amount  of  the  phosphate 
solution  to  a  small  test  tube  half  full  of  ammonium  molybdate  and 
gently  warming  the  mixture. 

Magnesium  chloride  that  contains  NH4C1  and  NH4OH  in 
excess  precipitates  MgNH4PO4  from  a  solution  of  a  phosphate. 
The  precipitate  is  crystalline  and  does  not  appear  immediately 
if  the  amount  of  phosphate  present  is  small  (see  page  74). 

DETECTION 

(Unless  phosphates  are  known  to  be  absent,  they  should  be 
tested  for  before  the  precipitation  of  the  ammonium  sulphide 
group  in  the  analysis  for  the  bases. 

If  members  of  Division  B  of  the  hydrogen  sulphide  group  are 
present,  they  must  be  removed  by  precipitation  with  H2S  and 
nitration  before  the  test  for  phosphates  is  made.  A  small  por- 
tion of  the  nitrate  is  then  evaporated  to  dryness  and  the  residue 
thus  obtained  is  used  instead  of  the  original  substance  in 
making  the  test.) 


SULPHATES  91 

Place  a  small  portion  of  the  original  solid  substance  in  an 
evaporator,  add  HNO%,  and  evaporate  to  clryness.  Add  a  little 
dilute  HNO3,  stir,  and  filter  if  the  residue  is  not  completely  dis- 
solved. Add  this  solution  to  5  cc.  of  (NH4)2MoO4  in  a  test 
tube,  warm  gently  (not  above  70°),  and  if  a  precipitate  does  not 
appear,  allow  to  stand  for  a  few  minutes. 

DISCUSSION 

The  method  of  analysis  of  the  ammonium  sulphide  group  of 
the  bases  must  be  modified  if  phosphoric  acid  is  present.  It  is 
therefore  necessary  to  test  for  this  acid  before  proceding  with 
the  analysis  of  that  group. 

It  is  also  necessary  to  remove  members  of  Division  B  of  the 
hydrogen  sulphide  group  because  arsenic  acid  gives  a  yellow 
precipitate  with  (NH4)2MoO4. 

Evaporation  with  strong  HNO3  is  necessary  in  order  to 
transform  salts  of  metaphosphoric  acid,  HPO3,  and  of  pyro- 
phosphoric  acid,  H4P2O7,  into  orthophosphoric  acid,  H3PO4, 
which  is  the  only  acid  of  phosphorus  that  gives  the  test  with 
(NH4)2Mo04. 

If  the  reagent  solution  of  ammonium  molybdate  is  too  highly 
heated,  a  white  precipitate  of  MoO3  is  produced. 

Ammonium  molybdate  gives  a  reddish  brown  precipitate  with 
ferrocyanides.  The  evaporation  to  dryness  with  HNO%,  if 
properly  performed,  will  destroy  ferrocyanides,  or  they  may  be 
precipitated  by  adding  ZnSO4  and  BaCl2  to  a  hot  solution  con- 
taining hydrochloric  acid.  The  BaSO4  is  a  heavy  precipitate 
and  carries  down  the  zinc  ferrocyanide. 

SULPHATES 

Most  sulphates  are  readily  soluble  in  water. 
The  sulphates  of  silver,  calcium,  strontium,  lead,  and  barium 
are  slightly  soluble  in  water,  the  solubility  decreasing  in  the 


92  THE  ACIDS 

order  given.  Most  basic  sulphates  are  insoluble  in  water. 
Lead  sulphate  is  soluble  in  ammonium  acetate,  forming  lead 
ammonium  acetate. 

If  a  sulphate  is  fused  with  a  carbonate  of  an  alkali  metal,  trans- 
position takes  place  and  an  alkali  sulphate  is  formed;  all  sulphates 
except  native  BaSO4  are  transposed  by  boiling  with  Na2CO3. 

The  sulphates  of  arsenic  and  antimony  are  rare  and  unstable. 
MnSO4  is  deliquescent. 

BaCl2  precipitates  BaS04  from  a  solution  of  a  sulphate.  The  precipi- 
tate is  white  and  is  insoluble  in  HC1. 

DETECTION 

Make  a  portion  of  solution  A  strongly  acid  with  dilute  HC1, 
filter  if  not  clear,  reject  the  precipitate,  and^to  the  warm1  solu- 
tion add  BaCl2  in  excess. 

DISCUSSION 

Dilute  and  not  concentrated  hydrochloric  acid  is  used  because 
the  latter  precipitates  barium  chloride  from  a  solution  of  that 
salt  of  the  concentration  here  employed  (see  list  of  reagents, 


CHROMATES 

Chromates  of  the  alkalies  and  of  magnesium  and  calcium 
are  easily  soluble  in  water  ;  strontium  chromate  is  less  soluble. 
All  chromates  are  colored  ;  those  most  commonly  met  with  are 
yellow  or  red.  A  water  solution  of  a  chromate  is  yellow,  that 
of  a  dichromate  reddish  yellow.  If  a  solution  of  a  chromate  is 
acidified  a  dichromate  is  formed,  while  if  a  solution  of  a  dichro- 
mate is  made  alkaline  a  chromate  is  formed. 

Chromates  are  transposed  by  boiling  with  Na2CO3. 

Most  reducing  agents  if  added  in  excess  to  an  acidified  solution  of  a 
chromate  reduce  the  latter  to  a  dark  green  chromic  salt,  such  as  CrCl3, 

1  See  Introduction,  page  15. 


CHROMATES  93 

Cr2(S04)3,  etc.,  according  to  the  acid  that  was  used  in  acidification,  If 
the  amount  of  acid  is  insufficient  to  form  the  chromium  salt,  a  precipita- 
tion of  Cr(OH)3  may  take  place. 

A  solution  of  a  chromate  will  be  reduced  when  treated  as  directed  in 
a,  6,  <?,  or  d : 

a.  Acidify  it  with  HC1,  heat  the  solution,  and  pass  through  it  hydro- 
gen sulphide  gas. 

b.  Acidify  with  HC1,  add  a  crystal  of  sodium  sulphite,  and  heat  to 
boiling. 

c.  Add  to  the  solution  a  considerable  amount  of  HCI  and  a  little  alcohol, 
C2H60,  and  boil  the  mixture.     The  alcohol  is  here  oxidized  and  acetalde- 
hyde,  C2H40,  is   formed.      This  last-named   substance  has  a  peculiar, 
characteristic  odor.     If  NH4OH  is  added  to  the  green  solution  of  CrCl3 
that  is  formed,  CrOH3  is  precipitated. 

d.  Acidify  the  solution  with  dilute  HN03  (see  list  of  reagents,  page  127) 
and  add  a  few  cc.  of  a  solution  of  El.      Iodine  is  set  free,  and  it  may  be 
extracted  by  adding  a  little  carbon  bisulphide,  CS2,  and  shaking  the  solu- 
tion.    The  iodine  will  be  taken  up  by  the  carbon  bisulphide  and  will 
impart  to  it  a  violet  color. 

Lead  ammonium  acetate  precipitates  PbCr04  from  a  solution  of  a 
chromate  that  has  been  acidified  with  acetic  acid.  The  precipitate  is 
yellow,  and  is  but  slightly  soluble  in  acetic  acid.  It  is  easily  soluble 
in  NaOH. 

AgNOg  when  added  to  a  solution  of  a  chromate  that  does  not 
contain  too  much  nitric  acid  precipitates  Ag2CrO4,  dark  red. 

For  the  reaction  of  H2O2  with  an  acidified  solution  of  a 
chromate,  see  pages  61  and  69. 

DETECTION 

. 

(The  presence  of  a  chromate  is  often  indicated  by  the  change 
of  color  that  takes  place  when  the  solution  is  treated  with  H2S 
in  the  analysis  for  the  bases.) 

If  solution  A  is  colorless,  chromates  are  absent.  If  solution  A 
is  not  colorless,  acidify  a  portion  with  acetic  acid,  filter  if  not 
clear,  reject  the  precipitate,  and  add  lead  ammonium  acetate. 
A  yellow  precipitate  proves  the  presence  of  a  chromate,  provided 
iodides  are  absent. 


94  THE  ACIDS 

DISCUSSION 

If  iodides  are  present  and  oxidizing  agents  other  than  a 
chromate  are  absent,  the  test  for  an  iodide  (page  100)  should  be 
made  without  the  addition  of  K2CrO4 ;  if  iodine  is  liberated, 
the  presence  of  a  chromate  is  proved. 

Under  proper  conditions  some  of  the  reactions  mentioned 
above,  such  as  that  with  alcohol  or  with  hydrogen  peroxide,  may 
be  used  to  advantage  for  the  detection  of  a  chromate. 

BORAXES 

Borates  of  the  alkalies  are  soluble  in  water.  Other  borates 
are  either  insoluble  or  not  readily  soluble  in  water,  but  are 
dissolved  by  dilute  inorganic  acids. 

Most  borates  are  readily  transposed  by  boiling  with  Na2CO3. 

When  a  few  drops  of  concentrated  H2SO4  and  several  cubic 
centimeters  of  methyl  alcohol  or  ethyl  alcohol  are  added  to  a 
solid  borate  and  the  mixture  is  warmed,  the  vapor  when  ignited 
burns  with  a  green  color.  This  experiment  may  be  performed 
in  an  ordinary  evaporating  dish,  but  the  test  is  much  more 
delicate  when  carried  on  in  the  apparatus  described  below, 
under  Detection. 

Turmeric  paper,  when  dipped  into  a  solution  of  a  borate  that 
has  been  slightly  acidified  with  dilute  HC1,  is  colored  reddish 
brown.  The  color  is  best  seen  when  the  paper  is  carefully 
dried.  KOH  changes  the  red  to  a  greenish  black  color. 

DETECTION 

Flame  test.  Place  a  small  portion  of  the  original  solid  sub- 
stance (see  Discussion)  in  a  60  cc.  flask,  provided  with  a  clean 
one-hole  stopper  through  which  passes  a  short  glass  tube  drawn 
out  at  the  outer  end  to  an  opening  about  2  mm.  in  diameter  and 
bent  at  an  angle  of  about  45°.  Add  to  the  contents  of  the 


BORATES  95 

flask  concentrated  H2SO4,  drop  by  drop,  until  in  excess.  Warm 
the  contents  of  the  flask  gently  for  a  minute.  Then  cool  the 
flask  and  add  2  cc.  of  methyl  alcohol.  Insert  the  stopper  carry- 
ing the  glass  tube,  place  the  flask  on  a  wire  gauze  over  a  Bunsen 
flame,  and  bring  the  contents  of  the  flask  to  gentl^^iling. 
As  soon  as  this  point  has  been  reached,  quickly  bring  the  flame 
of  the  burner  to  the  end  of  the  outlet  tube.  If  the  vapor 
burns  with  a  green  flame,,  the  presence  of  boron  is  proved. 
If  the  amount  of  boitate  present  is  small,  the  color  will  best 
be  observed  in  the  flashes  of  flame  that  appear  before  the 
vapors  are  evolved  in  such  quantity  that  they  burn  steadily. 

Turmeric  paper  test.  Place  a  portion  of  solution  C  in  an 
evaporator  and  heat  to  dryness.  If  solution  C  is  acid  it  should 
first  be  made  slightly  alkaline  by  the  addition  of  powdered 
Na2CO3.  (In  case  it  was  unnecessary  to  prepare  solution  C, 
evaporate  a  portion  of  solution  A  to  dryness*) 

To  the  dry  residue  add  dilute  HC1  until  slightly  acid.  Dip 
one  end  of  a  strip  of  turmeric  paper  into  the  solution  and  dry 
it  on  a  watch  glass  that  is  warmed  by  placing  it  on  a  water 
bath  or  high  above  a  Bunsen  flame.  If  a  borate  is  present,  the 
portion  that  has  been  immersed  in  the  solution  assumes,  when 
dry,  a  reddish  brown  color  which  changes  to  greenish  black 
when  touched  with  a  drop  of  KOH. 

DISCUSSION 

If  the  flame  test  is  to  be  used,  and  if  a  chlorate,  chromate,  or 
permanganate  is  present,  solution  C  should  be  employed.  A 
small  portion  of  this  solution,  if  acid,  is  rendered  alkaline  by 
the  addition  of  powdered  Na2CO3,  is  then  evaporated  to  dry- 
ness,  and  the  residue  is  used  for  the  flame  t^pt. 

The  reason  for  dipping  only  one  half  of  the  strip  of  turmeric 
paper  into  the  solution  is  to .  facilitate  the  recognition  of  the 
change  in  color,  for  when  only  traces  of  a  borate  «are  present, 
this  change  is  but  slight. 


96  THE  ACIDS 

A  chlorate,  chromate,  or  iodide  will  interfere  with  this  test 
for  a  borate.  The  first  two  will  have  been  reduced  by  Na2SO3 
in  the  preparation  of  solution  C.  If  an  iodide  is  present,  add 
HNO3  and  AgNO3  to  the  solution  —  after  reduction  by  Na2SO3, 
if  this  was  found  necessary  —  filter,  reject  the  precipitate,  make 
the  filtrate  alkaline  with  Na2CO3,  add  dilute  HC1  until  acid, 
and  test  with  turmeric  paper. 

Ferric  chloride  will  color  turmeric  paper  light  brown,  but 
when  the  dried  paper  is  then  touched  with  KOH  it  will  assume 
a  deeper  brown  color,  and  not  a  greenish  black. 

Dilute  hydrochloric  acid  must  be  used  in  this  test,  since  the 
concentrated  acid  turns  turmeric  paper  brown.  Turmeric  paper 
that  has  been  so  colored  turns  a  deeper  brown  when  touched 
with  a  drop  of  KOH. 


OXALATES 

Most  oxalates,  except  those  of  the  alkalies  and  magnesium, 
are  not  readily  soluble  in  water,  but  are  easily  soluble  in  dilute 
inorganic  acids. 

Most  oxalates,  with  the  exception  of  those  of  the  ammo- 
nium sulphide  group,  are  readily  transposed  by  boiling  with 
Na2CO3. 

When  an  oxalate  is  fused  with  NH4NO3,  oxidation  takes 
place  and  carbon  dioxide  and  water  are  formed. 

From  a  solution  of  an  oxalate  containing  acetic  but  no  other 
acid,  CaSO4  precipitates  CaC2O4,  white,  crystalline. 

DETECTION 

(Unless  oxalates  are  known  to  be  absent,  they  should  be 
tested  for  before  the  precipitation  of  the  ammonium  sulphide 
group l  in  the  analysis  for  the  bases.) 

1  See  page  66. 


TAETRATES   AND  ACETATES  97 

To  a  portion  of  solution  A  add  acetic  acid  to  strong  acid 
reaction,  filter  if  not  clear,  reject  the  precipitate,  and  add  CaSO4 
to  the  filtrate  and  warm. 

If  solution  A  is  colored  blue  by  a  copper  salt,  dilute  it  with  a 
little  water  before  it  is  acidified  and  boil  until  precipitation  of 
CuO  takes  place.  Filter,  acidify  the  filtrate  with  acetic  acid, 
and  add  CaSO4. 

TARTRATES   AND   ACETATES    (ORGANIC   MATTER) 

Evaporate  to  dryness  a  portion  of  solution  C  (or  of  solution 
A,  if  the  absence  of  chlorates,  chromates,  and  permanganates 
rendered  it  unnecessary  to  prepare  solution  C)  and  heat  the 
residue  in  a  porcelain  dish.  If  charring  occurs  or  smoky 
by-products  of  combustion  are  formed,  the  presence  of  organic 
matter  is  indicated. 

If  solution  C  is  acid,  it  must  be  made  alkaline  by  the  addition 
of  sodium  carbonate  before  it  is  evaporated  to  dryness. 

Solution  A  or  solution  C  may  contain  small  amounts  of  the 
salts  of  such  elements  as  copper.  When  this  is  the  case  the 
residue  will  assume  a  dark  color  when  heated. 

TARTRATES 

Tartrates  of  the  alkalies,  except  some  acid  tartrates,  are  readily 
soluble  in  water;  most  other  tartrates  are  insoluble  or  soluble 
only  with  difficulty. 

Tartrates  are  transposed  by  boiling  with  Na2CO3. 

When  a  tartrate  is  fused  with  NH4NO3  oxidation  takes  place 
and  carbon  dioxide  and  water  are  formed. 

When  a  tartrate  is  dissolved  in  concentrated  H2SO4  and  the 
solution  is  heated,  it  becomes  darkened  through  the  separation 
of  carbon.  If  the  heating  is  continued,  an  odor  of  burnt  sugar 
becomes  apparent  and  SO2  is  evolved. 


98  THE  ACIDS 

DETECTION 

(Unless  tartrates  are  known  to  be  absent,  they  should  be 
tested  for  before  the  precipitation  of  the  ammonium  sulphide 
group  1  in  the  analysis  for  the  bases.) 

Evaporate  to  dryness  a  portion  of  solution  C  rendered  alkaline 
by  the  addition  of  sodium  carbonate,  or  a  portion  of  solution  A 
if  it  has  not  been  necessary  to  prepare  solution  C.  Transfer  the 
residue  to  a  test  tube,  add  concentrated  H2SO4,  and  warm  gently. 
If  the  contents  of  the  test  tube  is  blackened  and  has  an  odor  of 
burnt  sugar,  SO2  also  being  evolved,  a  tartrate  is  present. 

DISCUSSION 

It  should  be  remembered  that  when  concentrated  sulphuric 
acid  is  highly  heated,  sulphur  trioxide  is  given  off.  The  stu- 
dent should  be  careful  not  to  confuse  the  odor  of  this  gas  with 
that  of  sulphur  dioxide. 

ACETATES 

Acetates  are  soluble  in  water.  When  neutral,  water  solutions 
of  certain  acetates  are  boiled,  hydrolysis  takes  place,  and  the 
hydroxides  of  the  metals  are  precipitated. 

All  acetates  are  transposed  by  boiling  with  Na2CO3. 

When  concentrated  H2SO4  is  added  to  an  acetate  and  the 
mixture  gently  warmed,  acetic  acid  is  liberated  and  may  be 
recognized  by  its  odor.  If,  in  this  experiment,  a  little  alcohol 
has  been  added,  there  is  formed  ethyl  acetate,  C2H5(C2H3O2), 
which  has  a  pleasant  ethereal  odor. 

When  FeCL  is  added  to  a  neutral  solution  of  an  acetate  the 
— — *> 

mixture  assumes  a  deep  red  color.  If  the  red,  neutral  solution 
is  diluted  and  boiled,  ferric  hydroxide  is  precipitated  and  the 
supernatant  liquid  becomes  colorless. 

1  See  page  66. 


IODIDES  99 

DETECTION 

If  an  acetate  has  not  already  been  detected  in  testing  for 
chlorates,  add  a  little  concentrated  H2SO4  and  a  few  drops 
of  alcohol  to  the  residue  left  on  evaporating  a  portion  of  solu- 
tion C  that  has  been  rendered  alkaline  by  the  addition  of  sodium 
carbonate,  or  of  solution  A  if  it  was  found  unnecessary  to  pre- 
pare solution  C ;  warm  gently  and  note  the  odor.  Confirm  by 
the  ferric  acetate  test. 

DISCUSSION 

Until  the  odor  of  ethyl  acetate  becomes  familiar  it  is  well  to 
make  a  comparative  test,  using  concentrated  sulphuric  acid  and 
alcohol  alone. 

In  making  the  ferric  acetate  test  it  is  desirable  that  the  solu- 
tion should  be  rendered  neutral  before  the  ferric  chloride  is 
added.  If  the  solution  is  acid,  ferric  acetate  may  not  be  formed  ; 
if  it  is  alkaline,  ferric  hydroxide  will  be  precipitated. 


IODIDES 

Iodides  are  easily  soluble  in  water  with  the  exception  of 
HgI2  and  the  iodides  of  the  hydrochloric  acid  group  of  metals. 
PbI2  is  very  slightly  soluble  in  cold  water,  but  is  readily  soluble 
in  hot  water. 

All  iodides  with  the  exception  of  the  insoluble  iodides  are 
easily  transposed  by  boiling  with  Na2CO3.  The  latter  are  easily 
transposed  upon  fusion  with  dry  Na2CO3. 

AgN03  precipitates  Agl  from  a  solution  of  an  iodide  acidified  with 
HN03.  The  precipitate  is  yellow,  and  is  insoluble  in  cold  NH4OH  or 
(NH4)2C03. 

Agl  is  not  decomposed  when  heated  to  dull  redness;  it 
is  decomposed  by  zinc  and  sulphuric  acid,  forming  zinc  sul- 
phate, hydriodic  acid,  and  metallic  silver.  If  the  reaction 


100  THE  ACIDS 

is  so  vigorous  that  the  acid  becomes  hot,  iodine  may  be 
liberated.  Silver  iodide  is  soluble  in  a  solution  of  potassium 
cyanide. 

Oxidizing  agents,  such  as  K2Cr04  or  chlorine  water,  liberate  iodine  from 
an  acidified  solution  of  an  iodide.  If  carbon  bisulphide  is  shaken  with  a 
solution  containing  free  iodine,  the  latter  is  extracted  and  imparts  a  violet 
color  to  the  CS2 . 

Concentrated  H2SO4  or  phenol-sulphonic  acid  liberates  iodine 
from  an  iodide. 

DETECTION 

Cool  a  portion  of  solution  B  and  acidify  with  HNO3.  (Use 
solution  A  if  it  has  been  unnecessary  to  prepare  solution  B.) 
Add  a  few  drops  of  K2CrO4  and  2  cc.  of  CS2.  Shake  vigor- 
ously and  note  the  color  of  the  CS2.  If  there  is  no  change,  add 
a  few  more  drops  of  K2CrO4  and  HNO3  and  shake  again. 

DISCUSSION 

If  in  the  preparation  of  solutions  for  the  analysis  of  the  bases 
a  residue  that  is  insoluble  in  water  and  acids  is  obtained,  an 
iodide  should  be  tested  for  as  directed  on  page  113. 


BROMIDES 

Bromides  are  easily  soluble  in  water  with  the  exception  of 
the  bromides  of  the  hydrochloric  acid  group  of  metals. 

All  bromides  except  the  insoluble  bromides  are  readily  trans- 
posed by  boiling  with  Na2CO3.  The  insoluble  bromides  are 
easily  transposed  upon  fusion  with  dry  Na2CO3. 

AgN03  precipitates  AgBr  from  a  solution  of  a  bromide  acidified  with 
HN03.  This  precipitate  is  light  yellow,  but  darkens  when  exposed  to 
the  light.  It  is  soluble  with  difficulty  in  cold  NH4OH ;  insoluble  in  cold 
(NH4)2C03. 

AgBr  is  not  decomposed  when  heated  to  dull  redness;  it  is 
decomposed  by  zinc  and  sulphuric  acid,  forming  zinc  sulphate, 


FERKOCYANIDES  101 

hydrobromic  acid,  and  metallic  silver.     Silver  bromide  is  solu- 
ble in  a  solution  of  potassium  cyanide. 

HBr  is  less  easily  oxidized  than  Hi ;  K2Cr04,  for  example,  does  not  set 
bromine  free  from  a  solution  of  a  bromide  slightly  acidified  with  HN03 ; 
chlorine  water  does  so,  however.  If  CS2  is  shaken  with  a  solution  con- 
taining free  bromine,  the  latter  is  extracted  and  imparts  a  yellow  or  red 
color  to  the  CS2  according  to  the  amount  of  bromine  present. 

Concentrated  H2SO4  or  phenol-sulphonic  acid  liberates  bro- 
mine from  a  bromide. 

DETECTION 

If  iodides  have  been  found  to  be  absent,  add  to  the  contents 
of  the  test  tube  used  in  the  test  for  an  iodide  a  few  drops  of 
chlorine  water  and  shake.  If  the  CS2  is  not  changed  in  color, 
add  a  few  drops  more  of  chlorine  water  and  shake  again. 

If  an  iodide  has  not  been  completely  decomposed  by  K2CrO4, 
iodine  will  be  liberated  when  chlorine  water  is  added.  There- 
fore, if  iodides  have  been  found  to  be  present,  filter  through  a 
wet  filter  the  contents  of  the  test  tube  used  in  the  test  for  an 
iodide.  With  the  filtrate  repeat  the  test  for  an  iodide  until  the 
iodide  is  completely  decomposed,  and  the  last  filtrate  when 
shaken  with  CS2  shows  no  trace  of  iodine.  Then  test  for 
bromides  with  chlorine  water  as  directed  in  the  preceding 
paragraph. 

DISCUSSION 

If  in  the  preparation  of  solutions  for  the  analysis  of  the  bases 
a  residue  that  is  insoluble  in  water  and  acids  is  obtained,  a 
bromide  should  be  tested  for  as  directed  on  page  113. 

FERROCYANIDES 

Ferrocyanides  of  the  alkalies  and  alkaline  earths  are  soluble 
in  water ;  most  other  f errocyanides  are  insoluble  in  water 
and  in  cold  acids.  The  ferrocyanides  of  the  alkalies  are  more 
insoluble  in  alcohol  than  the  corresponding  ferricyanides. 


102  THE  ACIDS 

Ferrocyanides  are  transposed  by  boiling  with  Na9CO3  or 
NaOH. 

When  potassium  ferrocyanide,  K4Fe(CN)6,  is  boiled  with  con- 
centrated H2SO4  it  is  decomposed  with  the  formation  of  CO, 
but  HCN  is  formed  if  the  acid  is  dilute.  When  heated  to 
dull  redness  Ag4Fe(CN)6  is  decomposed  with  the  formation  of 
metallic  silver. 

Ferrocyanides  in  acid  solution  are  easily  oxidized  to  ferri- 
cyanides  by  the  action  of  oxidizing  agents. 

When  FeSO4  is  added  to  an  acid  solution  of  potassium  ferro- 
cyanide there  is  formed  potassium  ferrous  ferrocyanide.  This 
is  a  whit&  precipitate,  but  it  is  so  easily  oxidized  that  under 
ordinary  conditions  of  preparation  it  has  a  light  blue  color  due 
to  the  formation  of  a  small  amount  of  ferric  ferrocyanide, 
Fe4[Fe(CN)6]8. 

From  an  acid  or  neutral  solution  of  a  ferrocyanide  FeCl3  pre- 
cipitates Fe4[Fe(CN)6]3,  dark  blue. 

DETECTION 

To  a  small  portion  of  solution  B  add  a  few  drops  of  FeCl3. 
A  dark  blue  solution  or  precipitate  indicates  a  ferrocyanide.  If 
it  has  been  unnecessary  to  prepare  solution  B,  use  a  portion  of 
solution  A  acidified  with  H^SO^1 

DISCUSSION 

It  should  be  borne  in  mind  that  a  ferricyanide  gives  a  deep 
blue  precipitate  with  ferrous  salts.  If,  therefore,  a  mixture 
contains  a  ferricyanide  together  with  a  substance  that  is  capable 
of  reducing  ferric  salts  to  the  ferrous  condition,  a  deep  blue 
precipitate  will  be  produced  even  in  the  absence  of  ferrocyanides. 

1  For  the  detection  of  the  bases  when  an  insoluble  ferrocyanide  or  ferri- 
cyanide is  present  see  Fresenius'  Qualitative  Analysis,  Wells's  translation, 
page  529  (1897). 


FERKICYANIDES  103 

FERRICYANIDES 

Ferricyanides  of  the  alkalies  and  alkaline  earths  are  .soluble 
in  water ;  most  other  f erricyanides  are  insoluble  in  w^r  and  in 
cold  acids.  Ferricyanides  of  the  alkalies  are  rnj#f%  soluble  in 
alcohol  than  the  corresponding  ferrocyanides. 

Ferricyanides  are  transposed  by  boUing  with  Na2CO3  or 
NaOH. 

When  potassium  ferricyanide,  K3Fe(CN)6,  is  boiled  with 
concentrated  H2SO4  it  is  decomposed  with  the  formation  of 
CO,  but  HCN  is  formed  if  the  acid  is  dilute.  When  heated 
to  redness  Ag3Fe(CN)6  is  decomposed  with  the  formation  of 
metallic  silver. 

Ferricyanides  in  alkaline  solution  are  easily  reduced  to  ferro- 
cyanides by  reducing  agents. 

From  an  acid  solution  of  a  ferricyanide  FeSO4  precipitates 
ferrous  ferricyanide,  Fe3[Fe(CN)6]2,  dark  blue. 

FeCl3  colors  a  solution  of  a  ferricyanide  dark  brown  or  yel- 
low according  to  the  degree  of  dilution. 

DETECTION 

Add  a  freshly  prepared  solution1  of  FeSO4  to  a  portion  of 
solution  B.  A  dark  blue  solution  or  precipitate  indicates  a 
ferricyanide.  (If  it  has  been  found  unnecessary  to  prepare 
solution  B,  use  a  portion  of  solution  A  acidified  with  H2SO4.)2 

DISCUSSION 

If  a  ferrocyanide  is  present,  make  the  solution  to  be  tested 
neutral,  concentrate  to  one-third  its  volume,  add  a  relatively  large 
quantity  of  alcohol,  and  allow  the  mixture  to  stand  for  half  an 
hour.  Filter  and  test  the  filtrate  as  directed  in  the  preceding 

paragraph. 

1  See  page  56.          2  See  note,  page  102. 


104  THE  ACIDS 

CHLORIDES 

All  chlorides  are  soluble  in  water,  except  the  chlorides  of  the 
hydrochloric  acid  group  of  metals  arid  a  few  basic  chlorides, 
such  as  BiOCl.  Lead  chloride  is  somewhat  soluble  in  cold 
water  and  is  easily  soluble  in  hot  water.  The  chlorides  of 
mercury,  silver,  lead,  cadmium^  cobalt,  barium,  strontium, 
potassium,  sodium,  and  ammonium  are  not  deliquescent;  with 
the  exception  of  a  few  basic  chlorides,  most  other  chlorides  are 
deliquescent. 

All  chlorides  are  readily  transposed  by  boiling  with  Na2CO3, 
with  the  exception  of  the  insoluble  chlorides.  The  latter  are 
easily  transposed  by  fusion  with  dry  Na2CO3. 

AgN03  precipitates  AgCl  immediately  from  a  solution  of  a  chloride 
acidified  with  nitric  acid.  This  precipitate  is  curdy,  white,  changing  to 
lavender  and  finally  to  black  on  exposure  to  the  light  ;  soluble  in  NH4OH 
or  (NH4)2C03,  probably  forming  AgCl-2NH3.  When  this  ammoniacal 
solution  is  acidified  with  nitric  acid  AgCl  is  precipitated. 

Silver  chloride  is  not  decomposed  when  heated  to  dull  red- 
ness ;  it  is  decomposed  by  zinc  and  sulphuric  acid,  forming  zinc 
sulphate,  hydrochloric  acid,  and  metallic  silver.  Silver  chloride 
is  soluble  in  a  solution  of  potassium  cyanide. 

DETECTION 

(If  a  white  precipitate  was  formed  in  the  test  for  the 
silver  nitrate  group  of  acids,  and  iodides,  bromides,  cyanides, 
and  ferrocyanides  have  been  found  to  be  absent,  a  chloride 
is  conclusively  proved  to  be  present,  and  no  further  test  is 
necessary.) 

To  a  portion  of  solution  B  add  HNO3,  and  then  AgNO3, 
filter,  and/,  wash  with  water  several  times  by  decantation. 
(If  it  ha^-been  found  unnecessary  to  prepare  solution  B,  use 
a  portion  of  solution  A  that  has  been  strongly  acidified 
with  HNO3.) 


CHLORIDES  105 

a.  If  iodides  are  present  and  bromides  are  absent,  digest  the 
precipitate  for  a  few  minutes  with  NH4OH  without  warming 
filter,  and  acidify  the  filtrate  with  HNO3.     A  white  precipitate 
proves  the  presence  of  a  chloride. 

b.  If   a   bromide    is   present,   whether   accompanied   by    an 
iodide  or  not,  digest  the  precipitate  for  a  few  minutes  with  a 
solution1  of  (NH4)2CO3  without  warming.     Filter  and  acidify 
the  filtrate  with  HNO3.     A  white  precipitate  proves  the  pres- 
ence of  a  chloride. 

c.  If  a  cyanide,  ferrocyanide,  or  ferricyanide  is  present,  dry 
the  precipitate  in  a  porcelain  crucible  and  heat  to  dull  redness. 
When  cool  add  a  piece  of  zinc  and  a  little  H2SO4,  and  allow  the 
reduction  to  proceed  for  at  least  half  an  hour,  adding  more  zinc 
and  sulphuric  acid  if  necessary.     Filter  and  to  the  filtrate  add 
a  few  drops  of  HNO3,  and  then  AgNO3.     Filter  and  test  the 
precipitate  as  directed  above  in  a  or  b. 

DISCUSSION 

If  in  the  preparation  of  solutions  for  the  analysis  of  the  bases 
a  residue  that  is  insoluble  in  water  and  acids  is  obtained,  a 
chloride  should  be  tested  for  as  directed  on  page  113. 

When  the  treatment  described  under  c  is  carried  out,  and 
iodides  and  bromides  have  been  found  to  be  absent,  the  produc- 
tion of  a  precipitate  on  the  addition  of  AgNO3  proves  that  a 
chloride  is  present. 

\  The  separation  of  iodides,  bromides,  and  chlorides  by  the  dif- 
|erent  solubilities  of  their  silver  salts  is  not  altogether  sharp, 


if  the  digestion  takes  place  in  the  cold.  In  a  and  6,  there- 
fore, something  more  than  a  faint  turbidity  should  be  produced 
upon  acidification  of  the  filtrate  before  the  presence  of  a  chloride 
is  considered  tp  be  conclusively  proved. 

JThe  shelf  reagent  contains  free  NH4OH  which  must  be  changed  to  the 
carbonate  by  passing  a  rapid  current  of  carbon  dioxide  through  the  solution 
for  ten  minutes  before  using  the  reagent  for  the  above  test. 


106  THE  ACIDS 

A  more  delicate  method1  for  the  detection  of  iodides,  bromides, 
and  chlorides  in  the  presence  of  one  another  consists  in  first 
oxidizing  the  iodide  with  ferric  sulphate  and  removing  the 
iodine  by  boiling,  then  oxidizing  the  bromide  with  potassium 
permanganate  and  removing  the  bromine  by  boiling,  and  finally 
testing  for  a  chloride  in  the  residual  solution. 


NITRATES 

All  nitrates  are  soluble  in  water,  except  a  few  basic  nitrates, 
such  as  BiONOg. 

The  nitrates  of  silver,  lead,  barium,  strontium,  potassium, 
sodium,  and  ammonium  are  not  deliquescent;  with  the  excep- 
tion of  a  few  basic  nitrates,  most  other  nitrates  are  deliquescent. 

All  nitrates  are  transposed  by  boiling  with  Na2CO3,  the 
insoluble  basic  nitrates  less  readily  than  the  others. 

When  a  little  concentrated  HN03  is  added  to  a  solution  of  FeS04  the 
ferrous  salt  is  oxidized,  and  NO  is  formed.  This  last-named  substance 
unites  with  the  unchanged  ferrous  sulphate  to  form  a  brown  unstable 
compound  (FeS04)2NO.  In  testing  for  a  nitrate,  concentrated  nitric  acid 
results  from  the  action  of  strong  sulphuric  acid  upon  th?  nitrate-s^jFhe 
test  is  performed  as  described  below. 

When  a  few  drops  of  a  solution  of  a  nitrate  are  evaporated  to  dryness 
on  a  porcelain  crucible  lid  and  the  cool  residue  is  treated  with  a  few 
drops  of  phenol-sulphonic  acid  and  then  gently  warmed,  picric  acid  (?)  is 
formed.  If  NH4OH  is  now  addf.  t#  the  cooled  solution,^  deep  yellow 
ammonium  picrate  is  formed.  Jr 


BKECT 


ION 

•  i 


Ferrous  sulphate  test.  Place  a  small  portion  of  solution  C  in 
a  test  tubej  adttW  c$i  of  concentrated  H2SO4  to  set  free  nitric 
acid  .from  any  nitrate  that  may  be  present,  and  cool  the  test 
tube.  Hold 'the  test  tube  in  a  slanting  position,  and  pour  in 
carefully  s^e^ral  cubic  centimeters  of  FeSO4  and  gently  tap  the 
<?  *  Edward  Hart ;  Am.  Chem.  /.,  6,  346  (1884). 


NITRATES  10T 

tube.     If  a  nitrate  is  present,  a  brown  ring  will  appear  wtr   j 
the  two  layers  of  liquid  meet.     The  ring  will  not  appear  im  ic- 
diately  if  but  traces  of  nitrate  are  present.     (Use  solution  A  ' 
this  test  if  it  was  found  unnecessary  to  prc  ion  C.)      lu. 

Phenol  sulphonic  acid  test.     Evaporate  a:         3-      >fsolut 
to  dryness  on  a  crucible  lid  and  test  with  j  -V"    >honicionC 

in  the  manner  described  above.     (If  soluti  i  it  sho-cid 

be  made  alkaline  by  the  addition  of  sodi  .ate  befuld 

being  evaporated  to  dryness.     Use  solutio  L  was  founcf 

unnecessary  to  prepare  solution  C.) 

DISCUSSION 

Ferrous  sulphate  test.  The  contents  of  the  test  tube  is  cooled 
after  the  addition  of  the  concentrated  sulphuric  acid  to  the  solu- 
tion to  be  tested  because  the  brown  substance,  (FeSO4)2NO,  is 
easily  decomposed  by  heat.  The  ferrous  sulphate  test  can  be 
used  only  when  iodides,  bromides,  ferrocyanides,  ferricyanides, 
chromates,  permanganates,  and  chlorates  are  not  present  in  the 
substance  under  examination,  or  have  been  removed.  If  these 
substances  are  present,  they  interfere  as  follows : 

Iodides  and  bromides  are  oxidized  by  the  concentrated  sul- 
phuric acid  with  the  liberation  of  iodine  and  bromine.  These 
elements  may  be  removed  by  adding  silver  acetate  or  silver 
sulphate  to  the  solution  and  filtering  off  the  precipitated  silver 
salts.  The  high  cost  of  the  reagents,  however,  makes  it  pref- 
erable to  use  the  phenol-sulphonic  acid  test  when  iodides  or 
bromides  are  present. 

Ferrocyanides  and  ferricyanides  form  blue  precipitates  with 
the  ferrous  sulphate.  Before  the  test  is  made  these  compounds 
may  be  removed  by  adding  sulphuric  acid  to  the  solution  to  be 
tested  until  the 'latter  is  slightly  acid,  and  then  adding  ferric 
chloride  and  ferrous  sulphate.  The  solution  is  then  heated  to 
boiling,  barium  chloride  is  added,  the  solution  is  filtered,  and 
the  nitrate  is  tested  for  a  nitrate.  The  barium  chloride  is 


108  THE  ACIDS 

added  to  form  barium  sulphate,  a  heavy  precipitate  which  will 
carry  down  the  ferrocyanides  and  ferricyanides. 

Chromates  are  reduced  by  ferrous  sulphate  to  chromium  sul- 
phate, which  will  appear  as  a  green  layer  at  the  place  where 
the  brown  ring  should  form,  and  will  therefore  obscure  the  test 
for  a  nitrate.  Chromates  are  reduced,  and  chromium  is  removed, 
in  the  preparation  of  solution  C. 

Permanganates  interfere  because  of  their  color.  These  also 
are  removed  in  the  preparation  of  solution  C. 

Chlorates  interfere  because  of  the  violent  reaction  that  takes 
place  when  they  are  treated  with  concentrated  sulphuric  acid. 
These  also  are  reduced  in  the  preparation  of  solution  C. 

Phenol-sulphonic  acid  test.  If  ferrocyanides,  ferricyanides, 
chromates,  permanganates,  and  chlorates  are  not  present  in  the 
original  substance  or  have  been  removed  as  directed  above,  this 
test  may  be  used  even  when  an  iodide  or  bromide  is  present. 


PAET   IV 

SYSTEMATIC  ANALYSIS  OF  A  SOLID  SUBSTANCE 

The  analyst  should  first  carefully  observe  the  physical  prop- 
erties of  the  substance,  noting  whether  or  not  it  is  crystalline, 
homogeneous,  deliquescent,  or  efflorescent,  and  whether  or  not 
it  possesses  the  characteristics  of  a  metal.  The  color  of  the 
substance  and,  if  it  is  clearly  a  mixture,  the  colors  of  the  differ- 
ent ingredients  should  be  observed  by  spreading  the  substance 
upon  a  sheet  of  white  paper  and  breaking  up  the  particles  by 
gentle  pressure  with  a  spatula. 

Whether  the  substance  uno^er  examination  contains  a  ferrous 
or  a  ferric  compound,  an  arsenious  or  an  arsenic  compound,  etc., 
must  be  ascertained  by  special  tests  before  the  material  has  been 
subjected  to  the  action  of  either  an  oxidizing  or  a  reducing  agent. 

I.    PREPARATION  OF  SOLUTIONS  FOR  THE  DETECTION 
OF  THE  BASES 

A.   THE  SUBSTANCE  is  NOT  A  METAL 

In  the  analysis  of  a  solid  that  may  be  a  mixture  of  several 
substances  the  attempt  is  first  made  to  separate  these  by  treating 
the  solid  successively  with  various  solvents. 

The  usual  order  in  which  the  solvents  are  applied  is :  (1)  hot 
water,  (2)  hydrochloric  acid,  (3)  nitric  acid,  and  (4)  aqua  regia. 

That  portion  of  the  substance  which  is  soluble  in  hot  water 
is  first  entirely  removed  by  that  solvent,  then  that  portion  of  the 
residue  which  is  soluble  in  hydrochloric  acid  is  removed,  etc. 

109 


110  ANALYSIS   OF   A   SOLID    SUBSTANCE 

It  is  desirable  to  bring  about  as  complete  separation  as  is 
possible  by  means  of  these  solvents.  The  residue  that  is  left 
undissolved  after  treatment  with  any  solvent  is  washed  thor- 
oughly with  water,  and  the  first  washings  are  added  to  the 
filtrate. 

If  there  is  doubt  as  to  whether  any  of  the  substance  has  been 
dissolved  by  a  solvent,  a  little  of  the  filtrate  is  evaporated  to 
dryness  in  a  porcelain  dish.  If  no  residue  remains,  the  solution 
may  be  discarded.  (A  very  slight,  impalpable,  brown  residue 
is  usually  due  to  dust  and  may  be  disregarded.) 

If  solution  takes  place  but  slowly,  as  is  the  case  when  certain 
oxides  are  treated  with  acids,  it  is  well  to  determine  when  the 
solvent  action  is  practically  complete.  This  is  done  by  treating 
the  washed  residue  a  second  time  with  the  same  solvent,  filter- 
ing, and  evaporating  a  small  portion  of  the  filtrate  to  dryness. 
If  an  appreciable  amount  of  residue  remains,  the  treatment  with 
the  solvent  should  be  repeated. 

Before  the  substance  is  treated  with  any  of  the  solvents  it 
should  be  thoroughly  mixed,  and  about  a  gram  of  it  should  be 
finely  pulverized  in  a  small  mortar  unless  grinding  produces 
an  explosion.  (This  should  be  ascertained  in  the  beginning  by 
grinding  a  very  small  portion  of  the  substance  in  a  mortar.) 

WATER  SOLUTION 

If  any  of  the  substance  has  dissolved  in  water,  test  the  solu- 
tion with  litmus  paper. 

a.  If  it  is  neutral  or  acid,  proceed  to  the  analysis  for  the  bases 
(page  21). 

b.  If  it  is  alkaline,  make  slightly  acid  with  HNOq.     If  there 
is  formed  a  precipitate  that  is  soluble  in  HNO3  (test  a  small 
portion),  dissolve  it  by  the  further  addition  of  that  acid.     If  the 
precipitate  does  not  dissolve,  filter,  wash  it  with  water,  and  treat 
it  as  if  it  were  a  solid  substance  presented  for  analysis  and  found 


PBEPABATION   OF   SOLUTIONS  111 

to  be  insoluble  in  water.  The  solution  acidified  with  HNO3, 
or  the  nitrate  from  the  precipitate,  is  analyzed  for  the  bases 
(page  21). 

HYDROCHLORIC   ACID   SOLUTION 

The  residue  that  may  remain  after  extracting  all  of  that  por- 
tion of  the  substance  which  is  soluble  in  water  is  treated  with 
dilute  HC1,  the  acid  being  heated  to  boiling  if  that  is  necessary 
to  effect  solution.  If  the  residue  seems  to  have  undergone  no 
change,  add  HCI  and  boil  until  the  solvent  has  no  further  action 
upon  the  residue. 

This  treatment  may  give  indications  as  to  the  presence  of 
certain  substances,  e.g.  : 

CO2  from  a  carbonate,  or  from  an  oxalate  and  an  oxidizing 
agent, 

HCN  from  a  cyanide, 
SO2  from  a  sulphite, 

H2S  from  a  sulphide,  or  from  a  sulphite  and  a  strong  redu- 
cing agent, 

Sulphur  from  a  sulphide  and  an  oxidizing  agent,  or  from  a 
sulphite  and  a  reducing  agent, 

Chlorine  from  an  oxidizing  agent, 
Bromine  from  a  bromide  and  an  oxidizing  agent, 
Iodine  from  an  iodide  and  an  oxidizing  agent, 
Oxides  of  nitrogen  from  a  nitrate  and  a  reducing  agent. 
If  the  treatment  with  hydrochloric  acid  seems  to  form  a  chloride 
of  one  of  the  members  of  the  hydrochloric  acid  group  of  metals,1 

1  When  hot  HCI  is  used  as  a  solvent,  the  substance  is  sometimes  transformed 
entirely  into  white  needle-like  crystals.  These  should  be  removed  and  dissolved 
in  hot  water  and  a  small  portion  of  the  solution  then  tested  for  lead.  If  lead  is 
found  to  be  present,  it  is  well  to  remove  it  at  this  point  as  completely  as  possible. 
This  may  be  done  by  evaporating  the  HCI  solution  to  very  small  bulk,  cooling  it, 
and  filtering  off  the  crystals  of  lead  chloride.  The  operation  should  be  repeated 
seVeral  times,  and  the  filtrate  finally  obtained  should  be  treated  as  directed  on 
page  38.  The  product  of  the  different  crystallizations  should  be  united,  dis- 
solved in  water,  and  the  water  solution  tested  for  the  bases,  especially  for 
mercuric  mercury. 


112  ANALYSIS   OF   A   SOLID   SUBSTANCE 

discard  the  mixture,  and  treat  a  fresh  portion  of  the  original 
substance  with  the  solvents  in  the  following  order:  (1)  hot 
water,  (2)  nitric  acid,  (3)  hydrochloric  acid,  etc. 

The  hydrochloric  acid  solution  can  contain  no  member  of  the 
hydrochloric  acid  group  of  metals  excepting  lead  unless  concen- 
trated acid  was  used.  Concentrated  hydrochloric  acid,  however, 
dissolves  silver  chloride  to  some  extent  and  any  of  this  substance 
that  may  be  present  in  solution  should  be  precipitated  as  com- 
pletely as  possible  by  diluting  the  solution  with  three  times  its 
volume  of  water.  After  the  precipitate  has  been  removed  by 
filtration  and  tested  for  silver,  the  analysis  should  be  proceeded 
with  as  directed  on  page  38. 

If  the  hydrochloric  acid  group  of  metals  is  absent,  treat  the 
solution  as  directed  on  page  38. 

NITRIC   ACID   SOLUTION 

Analyze  the  solution  as  directed  on  page  21,  first  diluting  it 
with  water  in  case  HNO%  has  been  used. 

NOTE.  —  If  there  is  present  a  sulphide  that  is  not  readily  acted  on  by 
hydrochloric  acid,  it  will  often  cause  the  separation  of  free  sulphur  in  the 
preparation  of  the  nitric  acid  or  aqua  regia  solution. 

AQUA  REGIA  SOLUTION 

Boil  to  expel  chlorine,  and  precipitate  the  hydrochloric  acid 
group  of  metals  as  completely  as  possible  by  diluting  the  solu- 
tion with  two  or  three  times  its  volume  of  water ;  then  proceed 
with  the  filtrate  as  directed  on  page  38. 

RESIDUE  INSOLUBLE  IN  WATER   AND   ACIDS 

This  may  contain  sulphur,  AgCl,  AgBr,  Agl,  PbSO4,  BaSO4 
(SrSO4,  CaSO4),  certain  oxides,  SiO2,  silicates,  and  some  salts  of 
hydroferrocyanic  and  hydroferricyanic  acids. 


PREPARATION   OF   SOLUTIONS  113 

Sulphur.  If  this  substance  is  set  free  from  a  compound  of 
the  element,  it  should  be  separated  by  filtration  and  dried.  It 
may  then  be  recognized  either  by  burning  it  to  SO2  or  by  con- 
verting it  into  sodium  sulphide  and  causing  this  to  act  upon 
metallic  silver  (see  page  87). 

Hal  ides  of  silver;  sulphates  of  lead,  barium  (strontium,  calcium); 
stannic  oxide.  Treat  the  insoluble  residue  with  a  concentrated 
solution  of  ammonium  acetate  containing  a  little  free  acetic 
acid,  boiling  the  mixture  vigorously  for  a  few  minutes.  Filter 
if  complete  solution  does  not  result,  and  test  the  filtrate  for 
lead  and  sulphuric  acid. 

If  an  insoluble  residue  still  remains  after  treatment  with 
ammonium  acetate  solution,  wash  it  thoroughly  with  hot 
water  until  the  wash  water  is  not  darkened  by  the  addi- 
tion of  ammonium  sulphide.  Divide  the  residue  into  two 
portions. 

Place  one  portion  in  a  porcelain  crucible,  add  metallic  zinc 
and  dilute  sulphuric  acid,  and  allow  the  action  to  continue  for 
at  least  fifteen  minutes.  The  halides  of  silver  will  be  reduced 
by  this  treatment,  the  silver  separating  in  metallic  form  and 
the  halogen  acid  being  set  free.  Filter  and  test  the  residue 
for  silver,  and  the  filtrate  for  HI,  HBr,  and  HC1.1 

Warm  the  other  portion  of  the  residue  insoluble  in  ammo- 
nium acetate  with  a  solution  of  potassium  cyanide.  This  treat- 
ment will  dissolve  the  halides  of  silver.  Filter.  (Do  not 
precipitate  the  halides  of  silver  from  the  filtrate  by  adding  an 
excess  of  an  acid,  because  HCN,  a  very  poisonous  gas,  would 
be  set  free.) 

1  It  must  be  borne  in  mind  that  when  silver  bromide  and  silver  iodide  are 
boiled  with  aqua  regia  they  are  changed  partly  or  completely  to  silver  chloride. 
If,  therefore,  insoluble  halides  of  silver  are  found  in  the  insoluble  residue,  a 
fresh  portion  of  the  original  substance  should  be  extracted  first  with  water  and 
then  with  cold  dilute  nitric  acid.  The  residue  thus  obtained  should  then  be 
treated  with  zinc  and  sulphuric  acid  and  tested  for  the  halogen  acids  as 
above  described. 


114  ANALYSIS   OF   A   SOLID   SUBSTANCE 

A  residue  that  now  remains  after  these  treatments  of  the 
insoluble  residue  with  ammonium  acetate  and  potassium  cyanide 
may  contain  sulphates  of  the  alkaline  earths  and  stannic  oxide. 
There  may  still  be  present  traces  of  lead  sulphate  and  the 
halides  of  silver.  These  last-mentioned  substances  must  be 
entirely  removed  before  proceeding  with  the  analysis.  To 
ascertain  whether  this  removal  has  been  complete,  place  a  very 
small  portion  of  the  residue  upon  a  porcelain  crucible  lid  and 
treat  it  with  a  drop  of  ammonium  sulphide.  If  it  is  darkened, 
extract  the  remainder  of  the  residue  once  more  with  ammonium 
acetate  and  test  a  particle  of  the  residue  again  with  ammonium 
sulphide.  If  it  is  still  blackened,  treat  the  residue  with  potas- 
sium cyanide  and  continue  these  alternate  treatments  until 
ammonium  sulphide  has  no  action  upon  the  residue. 

Now  dry  the  residue  and  pulverize  it  in  a  mortar.  Mix 
one-half  with  six  times  its  bulk  of  a  mixture  of  equal  parts 
of  Na2CO3  and  K3CO3.  Transfer  this  mixture  to  a  platinum 
crucible  and  fuse  it  over  a  blast  lamp  for  twenty  minutes,  or 
until  the  mass  is  in  quiet  fusion.  The  sulphates  of  barium, 
strontium,  and  calcium  are  by  this  treatment  converted  into 
carbonates,  and  soluble  sulphates  of  potassium  and  sodium  are 
formed.  The  fused  mass  is  allowed  to  cool  and  is  then 
extracted  with  hot  water,  the  solution  filtered,  and  the  filtrate 
tested  for  a  sulphate.  The  portion  remaining  upon  the  filter  is 
thoroughly  washed  with  hot  water,  and  is  then  extracted  with 
hydrochloric  acid  and  the  solution  tested  for  the  alkaline  earths. 

The  other  half  of  the  dry  pulverized  residue  is  mixed  with 
six  times  its  bulk  of  a  mixture  of  equal  parts  of  anhydrous 
sodium  carbonate  and  sulphur.  This  mixture  is  placed  in  a 
small  porcelain  crucible,  the  crucible  is  covered,  and  it  is  heated 
for  some  time  over  a  low  flame,  until  the  excess  of  sulphur  has 
been  distilled  off  and  burned.  The  crucible  is  then  allowed  to 
cool  and  the  substance  is  treated  with  warm  water.  The  solu- 
tion is  then  filtered,  aim  the  filtrate,  which  contains  sodium 


PREPARATION   OF   SOLUTIONS  115 

sulpliostannate,  Na2SnS3,  if  tin  was  present  in  the  insoluble  resi- 
due, is  acidulated  with  hydrochloric  acid.  This  will  precipitate 
stannic  sulphide,  which  should  be  identified  by  further  tests. 

Silicates.  Salts  of  silicic  acids  are  decomposed  when  treated 
with  concentrated  hydrofluoric  acid,  the  silicon  uniting  with 
fluorine  to  form  silicon  tetrafluoride,  SiF4,  which  is  a  gas.  This 
gas  is  decomposed  'by  water  with  the  formation  of  insoluble 
silicic  acid,  H4SiO4,  and  soluble  fluosilicic  acid,  H2SiF6: 

3  SiF4  +  4  H20  =  H4Si04  +  2  H2SiF6. 

If  a  silicate  is  present  in  the  substance  under  examination, 
there  will  appear  in  the  water  minute  white  particles  of  silicic 
acid;  and  if  the  water  is  then  carefully  evaporated  to  dryness 
on  a  piece  of  clean  platinum  foil,  a  white  residue  will  remain. 
If  this  is  then  highly  heated  with  the  Bunsen  burner,  the  silicic 
acid  will  be  changed  to  white  silicon  dioxide,  SiO2,  which  will 
not  be  volatilized  by  the  heating.  To  ascertain  by  the  aid  of 
the  above-described  reactions  whether  a  substance  contains 
silica  or  a  silicate,  proceed  as  follows : 

Place  about  1  cc.  of  concentrated  sulphuric  acid  in  a  clean, 
dry  platinum  crucible  and  add  carefully  about  one-fourth  of  a 
gram  of  the  substance  to  be  tested.  Warm  the  contents  of  the 
crucible  gently  and  allow  it  to  stand  until  effervescence  ceases. 
Allow  the  contents  of  the  crucible  to  cool  and  add  1  cc.  of 
pure  hydrofluoric  acid.  Cover  the  crucible  with  its  platinum 
cover  in  such  a  manner  that  an  opening  about  3  mm.  wide  is 
left  on  one  side,  and  hold  at  this  opening  a  platinum  wire  loop 
containing  a  drop  of  distilled  water.  Warm  the  contents  of 
the  crucible  gently  with  a  very  low  Bunsen  flame.  Observe 
whether  white  particles  of  silicic  acid  appear  in  the  drop  of 
water,  and  then  carefully  evaporate  the  drop  to  dryness  on  a 
piece  of  clean  platinum  foil  or  on  the  crucible  cover.  If  there 
remains  a  white  residue  that  is  not  volatilized  upon  high  heat- 
ing, silica  was  present  in  the  original  substance. 


116  ANALYSIS   OF   A   SOLID   SUBSTANCE 

Some  of  the  salts  of  silicic  acid  are  decomposed  by  strong 
acids,  others  are  not.  When  a  silicate  is  broken  down  by  an 
acid  the  bases  in  the  silicate  unite  with  the  acid  to  form 
salts  which  usually  are  soluble  in  water,  while  the  silicic  acid 
is  set  free  either  as  a  white  gelatinous  substance  or  as  a  gritty 
powder.  Silicates  that  are  not  attacked  by  strong  acids  may 
be  decomposed  by  fusing  the  finely  powdered  substance  with 
four  times  its  weight  of  a  mixture  of  equal  parts  of  sodium 
carbonate  and  potassium  carbonate,  allowing  the  mass  to  cool, 
and  then  treating  it  with  an  excess  of  hot  hydrochloric  acid. 
The  silicic  acid  set  free  by  either  of  these  treatments  is  not 
entirely  insoluble  in  water.  Since  its  presence  in  solution 
might  interfere  with  tests  for  some  of  the  bases,  it  should  be 
completely  removed  before  the  analysis  for  the  bases  is  under- 
taken. This  is  done  by  placing  the  hydrochloric  acid  solution 
in  an  evaporating  dish  and  evaporating  it  to  dryness  on  a  water 
bath,  continuing  the  heating  until  the  mass  is  dry  and  fumes'  of 
hydrochloric  acid  cease  to  be  given  off.  The  dry  residue  is  now 
moistened  with  HGI  and  is  warmed  for  a  few  moments.  Boiling 
water  is  then  added,  the  whole  is  heated  for  a  short  time,  the 
insoluble  SiO2  is  allowed  to  settle  and  is  removed  by  nitration. 
The  nitrate  is  then  analyzed  for  the  bases  in  the  usual  manner. 

The  insoluble  residue  may  contain  substances  other  than 
silica.  This  is  ascertained  by  tearing  off  the  point  of  the  filter 
containing  the  precipitate,  dipping  it  into  a  saturated  solution 
of  ammonium  nitrate,  placing  it  in  a  platinum  crucible,  and 
heating  it  over  the  flame  until  the  paper  is  entirely  consumed. 
Hydrofluoric  acid  is  then  added  to  the  residue ;  the  crucible  is 
placed  in  the  hood  and  is  gently  heated  until  the  liquid  is  driven 
off.  Two  or  three  repetitions  of  this  treatment  will  completely 
remove  all  of  the  silica,  and  the  residue  that  still  remains  should 
be  examined  as  directed  on  page  112. 

If  it  is  desired  to  detect  the  alkalies  that  are  present  in  the 
silicate,  this  may  be  done  by  decomposing  the  substance  by  means 


PREPARATION   OF   SOLUTIONS  117 

of  ammonium  chloride  and  calcium  carbonate1;  but  if  only  a 
qualitative  analysis  of  the  material  is  desired,  the  alkalies  may 
be  detected  with  greater  ease  and  accuracy  by  means  of  the 
spectroscope. 

Ferrocyanides  and  Ferricyanides.  For  the  detection  of  the 
bases  in  insoluble  ferrocyanides  and  ferricyanides,  the  student 
is  referred  to  Fresenius'  Qualitative  Analysis,  Wells's  transla- 
tion, page  529  (1897). 

B.   THE  SUBSTANCE  is  A  METAL  OR  AN  ALLOY 

Boil  1  gram  of  the  substance  with  20  cc.  HNO%  until  red 
fumes  are  no  longer  abundantly  given  off  and  the  action  ceases. 
Dilute,  filter,  and  analyze  the  filtrate  as  directed  on  page  21. 

Wash  the  residue  thoroughly  and  boil  with  HCI.  If  a  residue 
still  remains,  fuse  it  with  a  mixture  of  Na2CO3  and  sulphur  as 
directed  on  page  114. 

The  metals  considered  in  this  manual  are  acted  upon  by  HNO% 
and  are  changed  by  that  reagent  into  soluble  compounds,  with 
the  exception  of  tin  and  antimony,  which  are  changed  to  oxides. 
The  oxide  of  tin  is  insoluble  in  HNO&  and  the  oxide  of  antimony 
but  slightly  so. 

II.    PREPARATION  OF   SOLUTIONS  FOR   THE  DETECTION 
OF  THE  ACIDS 

In  the  analysis  of  a  solution  for  the  bases  the  different  groups 
of  metals  are  precipitated  by  the  successive  addition  of  various 
reagents.  This  mode  of  procedure  is  not  followed  in  the  analy- 
sis for  the  acids.  It  is  true  that  the  acids  are  classified  into 
groups  according  to  the  behavior  of  solutions  of  their  salts 
towards  certain  reagents,  but  these  reagents  are  used  solely  for 

1  Method  of  J.  Lawrence  Smith,  Crookes'  Select  Methods  in  Chemical  Analy- 
sis, page  26  (1894). 


118  ANALYSIS   OF   A   SOLID   SUBSTANCE 

the  purpose  of  determining  in  one  operation  whether  any  mem- 
bers of  a  group  are  present  in  the  solution.  If  the  addition  of 
such  a  group  reagent  to  the  solution  shows  that  all  the  members 
of  this  group  are  absent,  no  further  tests  need  be  made  in  this 
solution  for  the  acids  of  this  group.  If,  however,  the  group 
reagent  shows  that  some  members  of  the  group  are  present,  then 
special  tests  for  the  individual  members  of  this  group  must  be 
made. 

These  tests  for  the  different  acids  are  performed  with  sepa- 
rate portions  of  the  substance  under  examination  or  of  solutions 
prepared  from  it.  Such  a  method  of  procedure  renders  it 
possible  that  the  test  for  one  acid  may  be  interfered  with 
by  the  presence  of  other  acids  if  the  various  acids  are  not 
successively  removed,  as  is  the  case  in  the  analysis  for  the 
bases. 

It  is  impossible  to  prepare  a  single  solution  with  which  tests 
for  all  of  the  acids  may  be  performed.  If  the  substance  under 
examination  is  completely  soluble  in  water  or  in  some  one  acid, 
this  solution  may  be  often  used  in  the  tests  for  the  detection  of 
all  of  the  acids  except  the  solvent  and  those  acids  that  are 
decomposed  by  it.  It  is  usually  preferable,  however,  to  remove 
the  bases  that  are  present,  and  for  this  reason  the  original  sub- 
stance is  boiled  with  a  solution  of  sodium  carbonate.  Most 
salts  of  the  metals  are  transposed  by  this  treatment,  the  bases, 
with  the  exception  of  alkalies,  being  converted  into  insoluble 
carbonates,  and  the  acids  with  which  the  bases  were  originally 
united  being  changed  to  soluble  sodium  salts.  But  certain  salts 
are  not  transposed  by  this  treatment,  and  for  this  reason  it  is 
necessary  to  test  for  some  acids  either  in  a  solution  prepared  in 
a  different  manner  or  in  the  original  substance.  It  is  apparent 
that  the  original  substance  must  be  used  in  testing  for  carbon- 
ates, and  it  may  also  conveniently  be  employed  in  testing  for 
those  acids  whose  salts  liberate  gases  when  they  are  treated 
with  dilute  acids. 


PREPARATION   OF   SOLUTIONS  119 

In  the  analysis  for  the  acids  the  student  should  not  proceed 
blindly  to  the  preparation  of  the  sodium  carbonate  solution,  but 
should  carefully  consider  the  results  of  his  analysis  for  the  bases 
and  should  draw  conclusions  as  to  what  acids  may  be  present. 
The  intelligent  student  should  then  be  able  to  decide  whether 
or  not  it  is  possible  to  detect  the  acids  without  preparing  the 
sodium  carbonate  solution. 

SOLUTION  A 

Pulverize  about  a  gram  of  the  well-mixed  substance  in  a 
mortar,  unless  grinding  produces  an  explosion.  Treat  this  with 
15  cc.  of  a  saturated  solution  of  Na2CO3  and  boil  for  at  least 
fifteen  minutes,  unless  complete  solution  is  effected  in  a  shorter 
time.  Add  water  from  time  to  time  to  replace  that  which 
evaporates.  Finally  add  10  cc.  of  water  and  filter  if  a  residue 
remains,  rejecting  the  residue.  Mark  the  filtrate  "  Solution  A." 

If  solution  A  is  colored  purple  by  a  permanganate,  it  may  be 
decolorized  by  adding  crystallized  oxalic  acid  to  the  boiling  solu- 
tion. Filter  and  use  the  filtrate  where  solution  A  is  prescribed. 

SOLUTION   B 

If  the  acidification  of  solution  A  produces  a  yellow  precipi- 
tate, make  a  portion  of  solution  A  acid  with  dilute  H2SO4, 
filter,  reject  the  precipitate,  and  boil  the  filtrate  until  H2S  has 
been  entirely  expelled.  Mark  this  "  Solution  B." 

SOLUTION  C 

If  a  chlorate,  chromate,  or  permanganate  is  present,  place 
some  Na2SO3  in  a  porcelain  dish,  introduce  a  small  portion  of 
the  original  substance,  and  add  a  little  dilute  HC1.  Warm 
gently  toward  the  end  of  the  reaction.  After  the  reduction 
has  been  completed  make  sure  that  all  of  the  sulphite  has 
been  decomposed  and  that  SO2  has  Jbeen  expelled  from  the 


120  ANALYSIS   OF  A  SOLID   SUBSTANCE 

solution.  Filter  if  a  residue  remains,  and  reject  the  residue. 
If  the  nitrate  is  colored,  add  powdered  Na2CO3  until  effer- 
vescence ceases,  boil  for  a  few  minutes,  dilute  with  water,  and 
filter.  Mark  this  "  Solution  C." 

PRELIMINARY  TESTS   WITH   SOLUTION    A 

1.  Barium  Chloride  Group.  Acids  that  are  precipitated  from 
neutral  solutions  by  barium  chloride  :  (H2CO3),  (H2SO3),H2SO4, 
H2CrO4,  H3PO4,  H3BO3,  H2C2O4,  H2C4H4O6,  H3AsO3,  H3AsO4. 

Place  a  small  portion  of  solution  A  in  a  flask  and  add  HC1, 
drop  by  drop,  until  effervescence  ceases  and  the  solution  is  but 
slightly  acid.  Heat  the  solution  to  boiling,  filter  if  it  is  not 
clear,  cool  the  filtrate,  and  then  add  a  dilute  solution  of  NH4OH, 
drop  by  drop,  until  the  liquid  is  slightly  alkaline.  Then  add 
BaCl2  and  CaCl2. 

A  precipitate  appears  : 

A  member  of  the  BaCl2  group  of  the  acids  is  present. 

No  precipitate  appears  : 

a.  If  no  residue  was  left  in  the  preparation  of  solution  A 
with  Na2CO3,  members  of  the  BaCl2  group  (except  H2CO3 
and  H2SO3)  are  not  contained  in  the  substance  under  exami- 
nation. 

.  b.  If  a  residue  was  left  in  the  preparation  of  solution  A  with 
Na2CO3,  members  of  the  BaCl2  group  (except  H2CO3  and 
H2SO3)  are  not  present  in  solution  A ;  these  may,  however,  be 
present  in  the  original  substance,  since  certain  salts  are  not 
easily  transposed  by  Na2CO3. 

CaCl2  is  used  in  addition  to  BaCl2,  since  certain  calcium 
salts  of  the  acids  of  this  group,  such  as  the  oxalate  and  the 
tartrate,  are  less  soluble  than  the  corresponding  barium  salts. 

If  boric  acid  is  present  in  but  small  quantity,  it  may  not  be 
precipitated  by  BaCl2  and  CaCl2. 


PKEPAKATION   OF   SOLUTIONS  121 

The  presence  of  a  large  quantity  of  ammonium  salts  may 
prevent  the  complete  precipitation  of  some  of  the  members  of 
this  group. 


2.  Silver  Niirate  Group.  Acids  that  are  precipitated  fromjiitric 
acid  solution  by  silver  nitrate :  (H2S),  HI,  HBr,  (HCN),H4Fe(CN)6, 
H3Fe(CN)6,  HC1. 

To  a  portion  of  solution  B  add  5  cc.  of  HNO3  and  then 
AgNO3  in  excess.  If  there  is  no  precipitate,  members  of  the 
silver  nitrate  group  of  acids  (except  H2S  and  HCN)  are  absent. 
If  it  was  found  unnecessary  to  prepare  solution  B,  use  solution  A  , 
in  this  test,  making  it  acid  with  HNO3,  and  warming  it,  to 
expel  H2S  if  a  sulphide  is  present,  before  the  AgNO3  is  added. 

If  a  precipitate  is  formed,  note  its  color ;  filter,  place  in  a 
test  tube  the  filtrate,  or  the  solution  in  which  AgNO3  failed  to 
produce  a  precipitate,  and  add  a  dilute  solution  of  NH4OH, 
holding  the  tube  in  a  slanting  position.  If  a  precipitate  forms 
where  the  two  layers  of  liquid  meet,  its  color  may  indicate  the 
presence  of  certain  members  of  the  BaCl2  group  of  acids. 

Silver  salts  are  white  with  the  following  exceptions:  phos- 
j)hate,  arsenite,  iojlide,  —  yellow ;  bromide,  f errocyanide,  — 
pale  yellow ;  arsenate,  —  light  brown  ;  chromate,  —  dark  red ; 
ferricyanide,  —  orange  ;  sulphide,  —  black. 

It  must  be  remembered  that  in  the  preparation  of  solution  B, 
or  in  the  special  preparation  of  solution  A  for  this  test,  H2S 
and  HCN  have  been  expelled. 

If  but  a  faint  turbidity  is  produced  on  the  addition  of 
AgNO3,  it  is  usually  due  to  a  trace  of  a  chloride.  In  such  a 
case  the  other  members  of  this  group  need  not  be  tested  for. 

The  presence  of  a  large  quantity  of  ammonium  salts  may 
prevent  the  precipitation  of  traces  of  the  members  of  the  silver 
nitrate  group  of  acids. 


122  ANALYSIS   OF   A   SOLID   SUBSTANCE 

Silver  oxalate  and  silver  chromate  may  be  precipitated  by 
silver  nitrate  in  the  test  for  the  silver  nitrate  group  of  acids. 
These  precipitates  will  dissolve,  however,  when  more  nitric 
acid  is  added  and  the  mixture  is  warmed. 

After  these  preliminary  tests  have  been  performed,  special 
tests  should  be  made  for  such  acids  as  have  not  here  been 
shown  to  be  absent. 


III.    INTERPRETATION  OF  RESULTS 

All  of  the  data,  both  physical  and  chemical,  that  have  been 
secured  during  the  examination  of  the  substance  are  now  to  be 
utilized  in  drawing  conclusions  as  to  its  composition.  The  fol- 
lowing examples  are  introduced  to  illustrate  the  method  of 
reasoning  that  may  be  followed  in  the  interpretation  of  the 
results  of  the  analysis. 

1.  If  but  one  base  and  but  one  acid  have  been  found,  as,  for 
example,   cadmium  and  carbonic  acid,  the   chemical  evidence 
proves  that  the  substance  contains  cadmium  carbonate ;  but  it 
does  not  prove  that  the  substance  is  cadmium  carbonate  alone, 
for  there  may  also  be  present  metallic  cadmium,  cadmium  oxide, 
or  cadmium  hydroxide.     If  metallic  cadmium  is  present,  the 
silver-white  color  of  the  metal  will  be  observable.     If  cadmium 
oxide  is  present,  its  yellow  color  will  render  it  apparent.     If, 
however,  the  substance  is  to  all  appearances  a  homogeneous 
white  powder,  we  may  conclude  that  it  is  either  cadmium  car- 
bonate alone  or  a  mixture  of  cadmium  carbonate  and  cadmium 
hydroxide.     To  decide  which  is  the  case,  a  quantitative  analysis 
would  be  necessary,  since  we  have  no  simple  means  of  detecting 
the  presence  of  hydroxyl  groups. 

2.  The  successive  treatment  of  the  substance  with  different 
solvents  often  throws  light  upon  its  composition. 

Suppose,  for  example,  that  zinc,  sodium,  and  sulphuric  acid 
have  been  found  in  the  analysis  of  a  white  mixture.  If  both 


INTERPRETATION   OF   RESULTS  123 

zinc  and  sodium  were  found  in  the  water  solution,  we  may 
conclude  that  the  substance  is  a  mixture  of  zinc  sulphate  and 
sodium  sulphate ;  but  if  sodium  was  the  only  base  in  the  water 
solution,  and  if  the  zinc  was  found  only  in  the  acid  solution, 
it  is  evident  that  the  zinc  is  present  in  the  form  either  of  the 
oxide  or  of  the  hydroxide. 

Suppose,  again,  that  zinc  and  cadmium,  and  sulphuric  and 
carbonic  acids,  have  been  found  in  the  analysis  of  a  white  mix- 
ture. If  the  cadmium  was  found  in  the  water  solution,  and  the 
zinc  in  the  acid  solution,  the  substance  evidently  contains  cad- 
mium sulphate  and  zinc  carbonate  (oxide,  hydroxide) ;  but  if 
the  zinc  was  found  in  the  water  solution  and  the  cadmium  in 
the  acid  solution,  the  mixture  contains  zinc  sulphate  and  cad- 
mium carbonate  (hydroxide).  If  zinc  and  cadmium  were  found 
both  in  the  water  solution  and  in  the  acid  solution,  the  mixture 
consists  of  zinc  sulphate,  cadmium  sulphate,  zinc  carbonate, 
and  cadmium  carbonate  (zinc  oxide,  zinc  hydroxide,  cadmium 
hydroxide). 

3.  In  some  cases  it  is  difficult  to  determine  how  the  acids 
and  bases  are  combined.     If,  for  example,  a  mixture  contains 
two  such  salts  as  sodium  sulphate  and  ammonium  chloride,  the 
qualitative  analysis  will  fail  to  show  whether  these  compounds 
are  present  or  whether  the  mixture  contains  ammonium  sulphate 
and  sodium  chloride,  unless  the  chemist  is  able  to  pick  out 
crystals  of  the  different  substances  and  to  test  them  separately 
for  both  base  and  acid.     In  a  case  of  this  nature  it  is  customary 
to  report  the  acids  and  bases  as  combined  to  form  such  salts  as 
are  commonly  met  with  in  analytical  practice. 

4.  If  a  substance  contains  compounds  that  interact  when  an 
attempt  is  made  to  dissolve  the  mixture,  the  problem  becomes 
more  complex.     For  example,  when  water  is  added  to  a  mixture 
of  sodium  sulphate  and  barium  chloride,  these  substances  inter- 
act to  form  sodium  chloride  and  barium  sulphate.     In  such  a 
case  it  is  sometimes  possible  to  ascertain  the  composition  of  the 


124  ANALYSIS    OF   A   SOLID   SUBSTANCE 

original  mixture  by  mechanically  separating  the  ingredients 
and  analyzing  each.  If,  however,  the  substance  is  in  the  form 
of  a  finely  ground  powder,  it  is  difficult  to  do  this  and  it  is  con- 
sequently almost  impossible  to  ascertain  by  ordinary  means  how 
the  bases  and  acids  were  combined  in  the  original  substance, 
unless  a  product  of  the  interaction  differs  in  appearance  from 
the  original  mixture. 


APPENDIX 


LIST   OF  APPARATUS 

The  student  should  find  the  following  set  of  apparatus  in  his 
desk.  Any  deficiency  should  be  reported  at  the  first  laboratory 
practice  to  the  instructor;  otherwise  it  will  not  be  made  good. 
At  the  end  of  each  term  and  at  the  end  of  his  course  the  student 
must  clean  all  apparatus,  obtain  from  the  storeroom  anything 
lacking  from  the  set,  and  then  submit  his  desk  for  inspection. 

Glass : 

Beaker  :  50  cc.,  one 

Bottles  (glass-stoppered) :   60  cc.,  for  AgN08,  one 
250  cc.,  for  alcohol,  one 
Bottle  (small,  cork-stoppered  vial),  containing  0.5  gram  AgN08  in 

crystals 

Delivery  tube  (drawn  out  at  one  end)  :  one 

Flasks  (round,  flat  bottomed)  :  100  cc.  for  hydrogen  generator  (with 

rubber  stopper,  funnel  tube, 
delivery  tube,  rubber  connec- 
tion, and  platinum  foil),  one 
250  cc.  for  wash  bottle  (with  rubber 
stopper  and  two  glass  tubes), 
one 

60  cc.,  one 

Flasks  (Erlenmeyer,  conical):    60  cc.,  two 

100  cc.,  two 

300  cc.,  for  H2S  precipitation  (with 
one-hole  rubber  stopper,  bent 
tube,  and  rubber  tubing,  as 
described  on  page  38)  :  one 
125 


126 


APPENDIX 


Funnels  :  60  mm.,  three 

Loop  tube  (with  supporting  cork) :  (see  page  85),  one 

Plate :  10  x  10  cm.,  one 

Reduction  tubes  (of  difficultly  fusible  glass,  with  bulb  on  end,  for 
fusion  of  compounds  of  arsenic,  etc.)  :  three 

Rods  :  10  cm.,  two 

Test  tubes  :  15  cm.,  twelve 
10  cm.,  six 

Watch  glass :  55  mm.  diameter,  one 
Platinum  wire :  one 
Porcelain : 

Crucible  (30  mm.  diameter,  with  cover)  :  one 

Evaporator  (70  mm.  diameter)  :  one 

Mortar  and  pestle  :  one 

Bunsen  burner  (with  60  cm.  of  rubber  tubing)  :  one 
Forceps :  one 

Horn  spatula  (10  cm.  long)  :  one 
Iron  stand,  with  two  rings  :  one 
Iron  wire  gauze  :  one 
Iron  wire  triangle  :  one 
Sponge :  one 
Test  tube  brush :  one 
Test  tube  rack  :  one 
Filters  :  9  cm.  diameter,  one  hundred 

7  cm.  diameter,  fifty 
Litmus  paper  and  Turmeric  paper  : 
Safety  matches :    (no  other  kind  of 
match    should   be  used   in   the 
laboratory) 

"I       To  be  obtained  from  the  storeroom  on  special 
Platinum  spoon :  ,    ,         ,         ,  , 

>•  check  only  when  needed,  and  to  be  returned 

"Dl/i+^mim     »fii/»;KlA   •        !  <* 


To  be  obtained  on  order 
at  the  storeroom 


Platinum  crucible : 


J  immediately  after  use 


REAGENTS  IN  SOLUTION 


127 


REAGENTS  IN  SOLUTION 

It  is  often  necessary  in  qualitative  analysis  to  use  solutions  of 
acids  and  alkalies  of  certain  definite  strength.  It  is  convenient  to 
make  up  these  solutions  in  such  a  way  that  a  given  volume  of  the 
one  is  exactly  neutralized  by  the  same  volume  of  the  other,  or  by 
some  simple  multiple  of  that  volume. 

By  a  normal  solution  of  an  acid  is  meant  a  solution  containing  in 
one  liter  (a)  the  molecular  weight  of  the  acid  in  grams  if  the  acid 
contains  one  hydrogen  atom  replaceable  by  a  metal  ;  (b)  the  molecu- 
lar weight  of  the  acid  in  grams  divided  by  two  if  the  acid  con- 
tains two  atoms  of  hydrogen  thus  replaceable,  etc.  Thus :  one  liter 
of  a  normal  solution  of  HC1  contains  36.18  grams  of  hydrochloric 

acid ;  of  sulphuric  acid,  — ^ —  grams  of  the  acid,  or  48.67  grams. 

A 

A  normal  solution  of  an  alkali  is  one  made  up  of  such  strength 
that  any  volume  of  the  solution  is  exactly  neutralized  by  the  same 
volume  of  a  normal  solution  of  any  acid.  The  word  normal  is 
represented  by  the  letter  N;  2  N  means  that  the  solution  is  twice 
normal  strength  ;  3  N  that  it  is  three  times  normal  strength  ;  while 

N  N 

—  means    one-half  normal    strength;   —  means  one-third  normal 

Jj  O 

strength,  etc. 

The  further  explanation  of  normal  solutions  belongs  to  the  sub- 
ject of  quantitative  analysis. 


Acids : 

Acetic,  HC2H302  2  N 

Hydrochloric,  HC1,  dilute  2  N 

Hydrochloric,  HCI  6  N 
Hydrochloric,  HCI,  concentrated,      13  N 

specific  gravity  1.20 

Nitric,  HN08,  dilute  2  N 

Nitric,  HNOZ  6  N 

Nitric,  HNO8,  concentrated,  16  N 

specific  gravity  1.42 


Volume  of  acid  to  be  diluted 
to  one  liter 

138.5  cc.  of  80%  acid 
154.3  cc.  cone,  acid 
462.9  cc.  cone,  acid 


126.3  cc.  cone,  acid 
378.8  cc.  cone,  acid 


128 


APPENDIX 


TTXT/-V     r  •£ 

Nitric,  HN03,  fuming,  specific  grav- 

ity  1.60 

Sulphuric,  7/2$04 
Sulphuric,  H2S04,  concentrated, 

specific  gravity  1.84 
Hydrofluoric,  HF,  fuming,  chemi- 

cally pure,  40%. 
Tartaric,  H2C4H406 
Phenol-sulphonic 


Volume  of  acid  to  be  diluted 
to  owe  Kter 

6  N      166.03  cc.  cone,  acid 


36 


(see  page  130) 
(see  below) 

Grams  of  substance  in 
one  liter  Oy  soiution 


Alkalies  : 

Ammonium  hydroxide,  NH4OH  4 

266  cc.  concentrated  NH4OH  (sp.  gr. 

=  0.90)  diluted  to  1  liter  with  water 
Potassium  hydroxide,  KOH.    The  pure 

potassium  hydroxide  used  for  analyt- 

ical purposes  contains  about  20%  of 

water.     Hence  the  amount  necessary 

for  a  twice-normal  solution  is       2  N  =  (55.7  x  2)  x  |  =  139.25 
Sodium  hydroxide,  NaOH.     The  pure 

sodium  hydroxide  used   for  analyt- 

ical purposes  contains  about  10%  of 

water.      Hence   the    amount    neces- 

sary for  a  solution  four  times  nor- 

mal is  4  N  =  (39.76X4)X  Y-  =  176.70 

Salts  in  solution  and  other  reagents  in  liquid 
form: 

Ammonium  acetate,  NH4C2H302  300 

Ammonium  carbonate,  (NH4)2C03  (see  below) 

Ammonium  chloride,  NH4C1  100 

Ammonium  molybdate,  (NH4)2Mo04  (see  below) 

Ammonium  oxalate,  (NH4)2C204  •  H20  70 

Ammonium  polysulphide,  (NH4)2SX  (see  below) 

Ammonium  sulphide,  (NH4)2S  (see  below) 

•  Barium  carbonate,  BaC03  (in  suspension 
in  water) 


REAGENTS  IN  SOLUTION  129 

Grams  of  substance  in 
one  liter  of  solution 

Barium  chloride,  BaCl2  •  2  H20  60 

Bromine  water  (see  below) 

Calcium  hydroxide,  Ca(OH)2  (lime  water), 

saturated  solution 

Calcium  sulphate,  CaS04,  saturated  solution 
Carbon  bisulphide,  CS2 

Chlorine  water  (see  below) 

Ethyl  alcohol,  C2H5OH 

Ferric  chloride,  FeCl3  •  6  H20  90 

Ferrous  sulphate,  FeS04  •  7  H20  (see  below) 

Hydrogen  peroxide,  H202,  3%  solution 
Indigo  (see  below) 

Lead  ammonium  acetate, 

Pb(C2H302)2  +  NH4C2H302 

Magnesium  chloride,  MgCl2  •  6  H20  25 

Mercuric  chloride,  HgCl2  30 

Methyl  alcohol,  CH3OH 

Potassium  chromate,  K2CrO4  96 

Potassium  cyanide,  KCN  30 

Potassium  ferricyanide,  K3Fe(CN)6  30 

Potassium  ferrocyanide,  K4Fe(CN)6  •  3  H20  50 

Potassium  iodide,  KI  20 

Potassium  sulphate,  K2S04  85 

Potassium  sulphocyanate,  KCNS  50 

Silver  nitrate,  AgN03  42.5 

Sodium  acetate,  NaC2H302  135 

Sodium  ammonium  phosphate, 

NaNH4HP04  •  4  H20  70 

Sodium  carbonate,  Na2CO3  •  10  H20  200 

Sodium  cobaltic  nitrite,  Co(N02)3  •  3  NaN02  (see  below) 
Stannous  chloride,  SnCl2,  concentrated  (see  below) 

(used  in  the  Bettendorff  test  for  arsenic) 
Stannous  chloride,  SnCl2,  dilute  (see  below) 

Tartaric  acid,  H2C4H406  75 

Zinc  sulphate,  ZnS04  •  7  H20  140 


130  APPENDIX 

REAGENTS   IN   SOLID   FORM 

Aluminum  foil 

Ammonium  nitrate,  NH4N03 

Ammonium  sulphite,  (NH4)2S03  •  H2O 

Barium  hydroxide,  Ba(OH)2  •  8  H2O 

Borax,  Na2B407. 10 H20 

Calcium  carbonate,  CaC03  (marble) 

Copper  foil 

Cotton,  absorbent 

Ferrous  sulphate,  FeS04  •  7  H20 

Iron  filings 

Iron  foil 

Lead  acetate,  Pb(C2H302)2 . 3  H20 

Lead  dioxide,  Pb02 

Oxalic  acid,  H2C204-  2  H20 

Potassium  chlorate,  KC103 

Potassium  cyanide,  KCN 

Silver  nitrate,  AgN03 

Sodium  ammonium  phosphate,  NaNH4HPO4  •  4  H20 

Sodium  carbonate,  Na2C03 

Sodium  carbonate  and  potassium  nitrate,  Na2C03  -f-  KN03  (for 

fusions) 

Sodium  and  potassium  carbonates,  Na-jCOg  +  K2C03  (for  fusions) 
Sodium  hydroxide,  NaOH 
Sodium  sulphite,  Na2S03-7H20 
Sulphur,  powdered 
Tartaric  acid,  H2C4H4O6 

Tin  foil 

t 

Zinc  foil 

Zinc  granulated 


OTHER  SOLUTIONS 


131 


OTHER   SOLUTIONS   USED   IN   PRELIMINARY 

Aluminum  sulphate,  A12(S04)3- 18H2O 
Ammonium  nitrate,  NH4NOS 
Antimony  chloride,  SbCl3.     25  grams 
of  the  substance,  250  cc.  of  concen- 
trated HC1,  and  sufficient  water  to 
make  1  liter  of  solution 
Arsenic  pentoxide,  As205 
Arsenic  trioxide,  As203.     33  grams  of 
the  substance,  50  cc.  of  concentrated 
HC1,  and  sufficient  water    to  make 
1  liter  of  solution 

Bismuth  nitrate,  Bi(N03)3  •  5  H20.  (The 
solution  should  contain  some  free 
nitric  acid) 

Bismuth  chloride,  BiCl3.  (The  solution 
should  contain  some  free  hydro- 
chloric acid) 

Boric  acid,  H3B03,  saturated  solution 
Cadmium  chloride,  CdCl2  •  2  H20 
Calcium  chloride,  CaCl2  •  6  H20 
Chromium  sulphate,  Cr2(S04)3 
Cobalt  nitrate,  Co(N08)2 .  6  H2O 
Copper  chloride,  CuCl2  •  2  H2O 
Copper  sulphate,  CuS04  •  5  H20 
Lead  acetate,  Pb(C2H302)2  •  3  H2O 
Lead  nitrate,  Pb(N03)2 
Manganese  sulphate,  MnS04  •  7  H20 
Mercurous  nitrate,  HgN03  •  H2O 
Nickel  nitrate,  Ni(N03)2  •  6  H2O 
Potassium  bichromate,  K2Cr207 
Potassium  bromide,  KBr 
Sodium  phosphate,  secondary, 

Na2HP04.12H20 
Strontium  chloride,  SrCl2  •  6  H20 


EXPERIMENTS 

Grams  of  substance  in 
one  liter  of  solution 

25 

20 


38 


40 


26 

about  40 
25 
25 
30 
35 
20 
30 
90 
40 
35 
70 
35 
50 
30 

120 
30 


132  APPENDIX 


PREPARATION   OF   SPECIAL   REAGENTS 

Ammonium  carbonate.  Dissolve  200  grams  of  the  salt  in  a  mixture 
of  80  cc.  of  NH4OH  (sp.  gr.  0.90)  and  500  cc.  of  water,  and  after 
solution  is  complete,  dilute  with  water  to  1  liter. 

Ammonium  molybdate.  Dilute  100  cc.  of  NH4OH  (sp.  gr.  0.90)  with 
150  cc.  of  water  and  dissolve  in  this  solution  50  grams  of  molybdic 
acid.  Dilute  250  cc.  of  concentrated  nitric  acid  (sp.  gr.  1.42)  with 
500  cc.  of  water  and  pour  this  into  the  first  solution,  adding  it 
slowly  and  stirring  constantly.  Allow  to  stand  in  a  warm  place  for 
forty-eight  hours  and  decant  the  clear  supernatant  liquid  for  use. 

Ammonium  sulphide.  Pass  hydrogen  sulphide  through  750  cc.  of 
NH4OH  (sp.  gr.  0.90)  until  the  solution  is  saturated  with  the  gas. 
Then  add  to  the  solution  500  cc.  of  ammonium  hydroxide  (sp.  gr. 
0.90)  and  750  cc.  of  water. 

Ammonium  polysulphide.  Dissolve  a  little  sulphur  in  some  of  the 
ammonium  sulphide  solution  just  described. 

Bromine  solution.  Dissolve  50  grams  of  potassium  bromide  in 
500  cc.  of  water  and  add  10  cc.  of  bromine.  Shake  until  the 
bromine  is  dissolved. 

Chlorine  water.  Saturate  water  with  chlorine  gas.  The  solution 
decomposes  less  readily  if  kept  in  a  dark  place  or  in  a  bottle  of  dark- 
colored  glass. 

Chromic  acid  cleaning  mixture.  Dissolve  about  40  grams  of  pow- 
dered commercial  K2Cr207  in  about  150  cc.  of  warm  water.  Cool 
the  solution  and  pour  it  slowly  and  with  constant  stirring  into  about 
230  cc.  of  concentrated  H2S04.  Keep  in  a  500  cc.  wide-mouth  glass 
bottle.  The  crystals  of  chromic  acid  which  settle  on  standing  con- 
stitute the  active  constituent  of  the  cleaning  mixture,  and  therefore 
the  bottle  should  be  shaken  before  the  solution  is  introduced  into 
the  vessel  that  is  to  be  cleaned.  A  portion  of  it  that  has  been  used 
for  cleaning  may  be  poured  back  into  the  bottle,  provided  it  has  not 
been  diluted  with  water. 

Ferrous  sulphate.  Dissolve  140  grams  of  the  crystallized  salt  in 
about  800  cc.  of  water  to  which  5  cc.  of  concentrated  H2S04  has 
been  added.  Place  in  the  solution  some  pieces  of  metallic  iron 
(clean  tacks)  and  dilute  it  to  1  liter. 


PBEPABATION   OF    SPECIAL   KEAGEOTS         133 

Indigo  solution.  Place  five  parts  of  fuming  sulphuric  acid  in  a 
beaker  that  is  immersed  in  cold  water  and  then  add  slowly  and 
with  constant  stirring  one  part  of  finely  pulverized  indigo.  Cover 
the  beaker,  allow  the  mixture  to  stand  for  forty-eight  hours,  and 
then  pour  it  into  twenty  times  its  volume  of  water ;  stir  thoroughly 
and  filter. 

Phenol-sulphonic  acid.  Dissolve  24  grams  of  phenol  (white  crys- 
tals are  preferable)  in  a  mixture  of  148  cc.  of  concentrated  H2S04 
and  12  cc.  of  water.  The  solution  slowly  turns  dark  if  exposed  to 
the  light. 

Sodium  cobaltic  nitrite.  Dissolve  100  grams  of  sodium  nitrite  in 
300  cc.  of  water,  add  acetic  acid  to  slight  acid  reaction,  and  then  add 
10  grams  of  cobalt  nitrate.  Allow  the  solution  to  stand  for  several 
hours  and  filter  it  if  not  clear.  The  solution  decomposes  slowly, 
and  it  is  therefore  advisable  to  prepare  only  small  quantities  at 
a  time. 

Stannous  chloride,  concentrated  (to  be  used  in  the  Bettendorff  test 
for  arsenic).  Heat  an  excess  of  granulated  tin  in  hot  hydrochloric 
acid  (sp.  gr.  1.20)  until  the  solution  is  saturated.  Platinum  scrap 
may  be  added  for  the  purpose  of  hastening  the  action  of  the  acid 
on  the  tin.  Dilute  this  solution  with  four  times  its  volume  of 
water. 

Stannous  chloride,  dilute.  Dilute  the  preceding  solution  with  an 
equal  volume  of  water.  Place  this  solution,  together  with  a  few 
pieces  of  granulated  tin,  in  a  bottle  provided  with  a  two-hole  rubber 
stopper.  Through  one  opening  of  the  stopper  insert  a  siphon  tube 
provided  near  its  outer  extremity  with  a  glass  stopcock.  Through 
the  other  opening  in  the  stopper  insert  a  glass  tube  and  connect 
its  outer  end  with  a  carbon  dioxide  generator  in  such  a  manner  that 
when  stannous  chloride  is  drawn  off  through  the  siphon,  carbon 
dioxide  and  not  air  enters  to  take  its  place. 


134 


APPENDIX 


TABLE   OF  ATOMIC   WEIGHTS   OF  THE  ELEMENTS 
(F.  W.  CLARKE) 

1902 


Aluminum 26.9 

Antimony 119.5 

Argon 39.6 

Arsenic 74.45 

Barium        136.4 

Bismuth 206.5 

Boron 10.9 

Bromine 79.35 

Cadmium 111.55 

Cesium 131.9 

Calcium 39.8 

Carbon 11.9 

Cerium 138.0 

Chlorine 35.18 

Chromium 51.7 

Cobalt 58.55 

Columbium 93.0 

Copper 63.1 

Erbium 164.7 

Fluorine 18.9 

Gadolinium 155.2 

Gallium 69.5 

Germanium 71.9 

Glucinum 9.0 

Gold 195.7 

Helium 3.93 

Hydrogen 1.000 

Indium        ......  113.1 

Iodine    .......   125.89 

Iridium 191.7 

Iron        55.5 

Lanthanum 137.6 

Lead       .......  205.36 

Lithium 6.97 

Magnesium 24.1 

Manganese 54.6 

Mercury 198.50 


H  = 


Molybdenum     .....  95.3 

Neodymium      .....  142.5 

Nickel      .......  58.25 

Nitrogen       ......  13.93 

Osmium  .......  189.6 

Oxygen    .......  15.88 

Palladium     ......  106.2 

Phosphorus  ......  30.75 

Platinum      ......  193.4 

Potassium     ......  38.82 

Praseodymium       .     .     .     .139.4 

Rhodium       ......  102.2 

Rubidium     .....     .84.75 

Ruthenium  ......  100.9 

Samarium     ......  149.2 

Scandium     ......  43.8 

Selenium      .     .     .     ...  78.6 

Silicon     .......  28.2 

Silver       ...     .     .     .     .  107.11 

Sodium    .......  22.88 

Strontium     ......  86.95 

Sulphur   .......  31.83 

Tantalum     ......  181.5 

Tellurium     ......  126.1 

Terbium       ......  158.8 

Thallium      ......  202.61 

Thorium       ......  230.8 

Thulium       ......  169.4 

Tin      ........  118.1 

Titanium      .     ...     .     .     .  47.8 

Tungsten      ......  182.6 

Uranium       ......  237.8 

Vanadium  '  ......  51.0 

Ytterbium    ......  171.9 

Yttrium  .......  88.3 

Zinc    ........  64.9 

Zirconium          .  89.7 


INDEX 


Abbreviations    of   titles  of    journals, 

136. 

Acetates,  detection  of,  99. 
Acetates,  reactions  of,  84,  98. 
Acids,   preparation  of    solutions    for 

detection  of  the,  117. 
Acids,  the,  83. 

Alkalies,  detection  of  the,  80. 
Alkalies,  the,  79. 
Alkali  metals,  detection  of,  75. 
Alkaline  earths,  71. 
Aluminum,  detection  of,  64. 
Aluminum,  reactions  of  salts  of,  60. 
Ammonia,  79. 

Ammonia,  detection  of,  80.      ^ 
Ammonium  carbonate  group,  71. 
Ammonium  carbonate  group,  analysis 

of,  74. 

Ammonium  salts,  reactions  of,  79. 
Ammonium  sulphide  group,  63. 
Ammonium  sulphide  group,  analysis 

of,  62,  64. 
Ammonium  sulphide  group,  analysis 

of,  when  a  phosphate  is  present,  64, 

70. 
Ammonium  sulphide  group,  analysis 

of,  when  oxalates  and  tartrates  are 

present,  62,  64. 
Antimony,  behavior  of,  in  Bettendorff 

tests  for  arsenic,  36,  47. 
Antimony,  behavior  of,  in  Gatehouse 

test  for  arsenic,  36. 
Antimony,   Bettendorff   test  for,  36, 

47. 


Antimony,  detection  of,  44,  47,  49. 
Antimony,  Gutzeit  test  for,  36,  47. 
Antimony,  reactions  of  compounds  of, 

34. 

Apparatus,  list  of,  125. 
Appendix,  125. 
Arsenic,  Bettendorff  tests  for,  32,  34, 

46. 

Arsenic,  detection  of,  44,  45,  49. 
Arsenic,  Gatehouse  test  for,  32,  34, 

46. 

Arsenic,  Gutzeit  test  for,  31,  45. 
Arsenic,    reactions  of  compounds  of, 

30. 

Arsenic,  Reinsch  test  for,  32,  34,  46. 
Atomic  weights  of  the  elements,  134. 

B 

Barium,  detection  of,  75. 
Barium,  reactions  of  salts  of,  71. 
Bases,   preparation    of    solutions    for 

detection  of  the,  109. 
Bases,  the,  18. 
Bettendorff    test    for    antimony,    36, 

47. 
Bettendorff  tests  for  arsenic,   32,   34, 

46. 
Bettendorff  tests  for  arsenic,  behavior 

of  antimony  in,  36,  47. 
Bismuth,  detection  of,  27. 
Bismuth,  reactions  of  salts  of,  26. 
Borates,  detection  of,  94. 
Borates,  reactions  of,  94. 
Bromides,  detection  of,  101. 
Bromides,  reactions  of,  84,  100. 


139 


140 


INDEX 


Cadmium,  detection  of,  27. 
Cadmium,  reactions  of  salts  of,  25. 
Calcium,  detection  of,J75._ 
Calcium,  reactions  of  salts  of,  73. 
Carbonates,  detection  of,  85. 
Carbonates,  reactions  of,  84,  85. 
Chlorates,  detection  of,  83. 
Chlorates,  reactions  of,  83. 
Chlorides,  detection  of,  104. 
Chlorides,  reactions  of,  84,  104. 
Chromates,  detection  of,  93. 
Chromates,  reactions  of,  92. 
Chromium,  detection  of,  64,  69. 
Chromium,  reactions  of  salts  of,  60. 
Cobalt,  detection  of,  64. 
Cobalt,  reactions  of  salts  of,  54. 
Colloids,  12. 
Conversion  factors,  138. 
Copper,  detection  of,  27,  29,  43. 
Copper,  reactions  of  salts  of,  24. 
Cyanides,  detection  of,  89. 
Cyanides,  reactions  of,  84,  88. 


Detection  of  the  acids,  preparation  of 

solutions  for  the,  117. 
Detection  of  the  bases,  preparation  of 

solutions  for  the,  109. 

E 

Earths,  the  alkaline,  71. 
Equations,  7. 

Equations,  balancing  of,  8. 
Equations  involving  oxidation,  8. 
Evaporation,  17. 

F 

Ferricyanides,  detection  of,  103. 
Ferricyanides,  detection  of  the  bases 

in   insoluble,  117. 
Ferricyanides,  reactions  of,  84,  103. 


Ferrocyanicles,  detection  of,  102. 
Ferrocyanides,  detection  of  the  bases 

in   insoluble,  117. 

Ferrocyanides,  reactions  of,  84,  101. 
Filtration,  14. 


Gatehouse    test  for   arsenic,    32,   34, 

46. 
Gatehouse  test  for  arsenic,  behavior 

of  antimony  in,  36. 
Gutzeit  test  for  antimony,  36,  47. 
Gutzeit  test  for  arsenic,  31,  45. 


H 

Hydrochloric  acid  group,  18. 
Hydrochloric  acid  group,  analysis  of, 

21. 

Hydrogen  generator,  30. 
Hydrogen  sulphide  group,  23. 
Hydrogen  sulphide  group,  analysis  of 

Division  A  of,  27. 
Hydrogen  sulphide  group,  analysis  of 

Division  B  of,  44,  48. 
Hydrogen  sulphide  group,  Division  A 

of,  23. 
Hydrogen  sulphide  group,  Division  B 

of,  30. 

Hydrogen   sulphide  group,  precipita- 
tion of,  38. 
Hydrogen  sulphide  group,  separation 

of  Divisions  A  and  B  of,  39. 


Insoluble  substance,  analysis  of,  112. 

Interpretation  of  results,  122. 

Introduction,  1. 

Iodides,  detection  of,  100. 

Iodides,  reactions  of,  84,  99. 

Iron,  detection  of,  64. 

Iron,  reactions  of  salts  of,  55,  56. 


INDEX 


141 


Journals,   abbreviations  of  titles    of, 
136. 


Lead,  detection  of,  21,  27. 
Lead,  reactions  of  salts  of,  19. 
List  of  apparatus,  125. 
List  of  reagents,  127. 
Loop  tube,  85. 


Potassium,  reactions  of  salts  of,  79. 
Precipitates,     action    of      excess    of 

reagent  upon,  13. 
Precipitate,  solution  of  a,  16. 
Precipitates,  solubility  of,  11. 
Precipitates,  washing  of,  16. 
Precipitation,  11. 
Precipitation,  conditions  affecting  the 

completeness  of,  11. 
Pseudo-solutions,  12. 


Magnesium,  detection  of,  75. 
Magnesium,  reactions  of  salts  of,  73. 
Manganese,  detection  of,  64,  69. 
Manganese,  reactions  of  salts  of,  58. 
Mercury,  detection  of,  21,  27. 
Mercury,    reactions   of  salts    of,    18, 

23. 
Metric  measures,  table  of,  137. 


N 

Nickel,  detection  of,  64. 
Nickel,  reactions  of  salts  of,  53. 
Nitrates,  detection  of,  106. 
Nitrates,  reactions  of,  84,  106. 


Organic  matter,  destruction  of,  62,  66. 
Organic  matter,  detection  of,  97. 
Oxalates,  detection  of,  96. 
Oxalates,  reactions  of,  84,  96. 
Oxidation  and  reduction,  8. 


Periodic  System,  135. 
Phosphates,  detection  of,  90. 
Phosphates,  reactions  of,  90. 
Potassium,  79. 
Potassium,  detection  of,  80. 


Qualitative  analysis,  definition  of, -1. 
Quantitative  analysis,  definition  of,  1 . 


Reactions,  3. 

Reactions,  conditions  affecting,  5. 

Reactions,  reversible,  4. 

Reagents,  list  of,  127. 

Reinsch  test  for  arsenic,  32,  34,  46. 

Results,  interpretation  of,  122. 

Reversible  reactions,  4. 


Silicates,  115. 

Silver,  detection  of,  21,  22. 

Silver,  reactions  of  salts  of,  18. 

Sodium,  79. 

Sodium,  detection  of,  80. 

Sodium,  reactions  of  salts  of,  79. 

Solid  substance,   systematic    analysis 

of,  109. 

Solution  of  a  precipitate^  16. 
Solutions  for  the  detection  of  the  acids, 

preparation  of,  117. 
Solutions  for  the    detection    of    the 

bases,  preparation  of,  109. 
Strontium,  detection  of,  75. 
Strontium,  reactions  of  salts  of,  72. 
Sulphates,  detection  of,  92. 
Sulphates,  reactions  of,  91. 


142  INDEX 

Sulphides,  detection  of,  87.  Tartrates,  reactions  of,  84,  97. 

Sulphides,  reactions  of,  84,  85.  Tin,  detection  of,  44,  47^  49. 

Sulphites,  detection  of,  88.  Tin,  reactions  of  compounds  of,  36. 

Sulphites,  reactions  of,  84,  87. 

Systematic  analysis  of  a  solid  sub-  ^ 

stance,  109. 

Washing  of  precipitates,  16. 


Table  of  metric  measures,  137.  Zinc,  detection  of,  64. 

Tartrates,  detection  of,  98.  Zinc,  reactions  of  salts  of,  59. 


LABORATORY  RECORD 


(Remove  this  sheet  and  paste  it  in  the  front  of  the  Laboratory  Notebook) 

The  results  of  an  experiment  should  be  recorded  when  the  experiment  is 
performed. 

PRELIMINARY  EXPERIMENTS 

Enter  on  the  left-hand  page  of  the  notebook,  in  the  form  of  an  equation, 
every  reaction  that  can  thus  be  expressed.  Enter  on  the  right-hand  page  a 
description  of  the  character,  solubility,  and  color  of  the  precipitate,  and  any 
change  in  color  that  the  solution  may  undergo. 

ANALYSIS   OF  A   KNOWN  MIXTURE 

Enter  on  the  left-hand  page  the  successive  chemical  changes  undergone  by 
each  member  of  the  group  in  the  course  of  the  analysis.  The  right-hand  page 
should  be  kept  as  in  Analysis  of  an  Unknown  Mixture  (see  below). 

EXAMPLE  (Left-Hand  Page) 


Pb(N03)2] 
HgN03      [  + 
AgN03 


HC1 


PbCl2 
HgCl 
AgCl 

HgCl 


+  Hot  Water 


NH4OH 
AgCl  •  2  NH3  +      HN03 
PbCl2      +     K2Cr04 


PbCr04 
PbCl2 


NaOH 
KI 


HgNH2Cl 

Agd-2NH, 

AgCl 

PbCr04 

Pb  (ONa)2 
PbI2 


ANALYSIS  OF  AN  UNKNOWN  MIXTURE 

Keep  a  record  of  your  work  as  illustrated  below,  entering  on  the  right-hand 
page  your  observations  together  with  statements  as  to  what  the  observed  phe- 
nomena indicate  or  prove  concerning  the  presence  or  absence  of  the  members 
of  the  group. 

EXAMPLE 

Left-Hand  Page  Right-Hand  Page 

Unknown  sol.  +  HC1 — >Ppt.  1  +  Sol.  1. 
Ppt.  1  +  hot  water 
Ppt.  2  +  NH4OH 
Sol.  3  +  HNO3 
Sol.  2  +  K2CrO4 


Ppt.  5  +  NaOH 
Sol.  2  +  KI 


.Ppt.  2  +  Sol.  2. 
Ppt.  3  +  Sol.  3. 

•  Ppt.  4. 

•  Ppt.  5. 

•  Sol.  4. 
.Ppt.  6. 


White  ppt.  indicates  HC1  group. 

White  res.  indicates  Ag  or  Hg,  or  both. 

Black  res.  proves  Hg. 

White  ppt.  proves  Ag. 

Yellow  ppt.  indicates  Pb. 

Soluble  ;  confirms  Pb. 

Yellow  ppt.  soluble  in  hot  water,  and 

crystallizing  in  shining   plates    on 

cooling,  proves  Pb. 


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